CHAPTER • 9 Minnesota's Mineral Resources IN MINNESOTA the production of iron ore is far more valuable economically than the total of all other mineral products, but im­ portant industries are based on Minnesota's other geological forma­ tions as well. Architectural, monumental, and structural stone are produced from granite, limestone, dolomite, and other Minnesota rocks. Gravel and sand are excavated and processed, and clay is used for many ceramic products. :Manganese in important amounts occurs in the iron ores of the Cuyuna district. Finally, although they are often not thought of as mineral products, two of our most im­ portant mineral resources are water and soil. The iron ores and mining operations of the Mesabi, Vermilion, and Cuyuna iron-bearing districts and of the southeastern lYlinnesota counties will be discussed in detail in later chapters, but a few sta­ tistics on Minnesota's iron ore industry may remind us how impor­ tant this geological heritage is. The following is an estimate of Min­ nesota's iron ore reserves, made on lYlay 1, 1961: Gross Tons Mesabi Range 500,799,179 Vermilion Range 9,755,974 Cuyuna Range 36,530,000 Fillmore County 'il,860,337 Total iron ore 549,945,490 172 MI NESOTA'S MINERAL RESOURCES The total production of iron ore in Minne ota to January 1, 1962, was 2,529,737,553 tons. Total taxes paid on iron ore to January 1, 1961 , were approximately $1,257,448,400, a very important source of funds for the state government. Slightly over 60 per cent of the total iron ore produced in the United States has come from l\1inne- ota. It is not surprising, therefore, that none of l\1innesota's other mineral resources approaches the iron mines in economic impor­ tance. The quarrying industry, however, has been important in Min­ nesota for over 50 years (Figures 81, 82, and 83). Many beautiful granites and dolomites produced in the state find a market over prac­ tically the entire nation and have been used in many well-known buildings. The first record of the quarrying in Minnesota of dimension stone, Figure 81. Air view of granite quarry southwest of St. Cloud, Stearns County. (Courtesy of Cold Spring Granite Co.) 173 Figure 32. Quarry in granite. Isle, Mille Lacs County. (Courtesy of Cold Spring Granite Co.) 174 MINNESOTA'S MINERAL RESOURCES Figure 83. Quarry in Oneota Dolomite, Minnesota River Valley at Mankato. or building stone cut to different sizes, dates back to 1820, when limestone was quarried locally for part of old Fort Snelling. Later, in IS35, General Sibley quarried limestone for a home at Mendota, across the river from Fort Snelling. Limestone quarries were oper­ ated at Stillwater, Mankato, and Winona as early as 1854. Granite was first quarried at St. Cloud in 1868, and within a few years thou­ sands of tons were shipped to widespread points. The term "granite" as used commercially includes granites, gneisses, diorites, gabbros, and other igneous rocks. Rough dimension stone for large buildings furnished the first important market, but beginning in 1886 paving blocks were also in demand. Quartzite was first quarried at New Dim in 1859, and somewhat later at Pipestone and elsewhere in southwestern Minnesota. The very productive dolomite quarries at Kasota were opened in 1868 and have continued as large producers of a variety of stone to the present time. In the St. Cloud region, the quarried rocks are granites and re- 175 MINNESOTA'S ROCKS AND WATERS lated rock types. They may be grouped into three major types, namely, pink granite, red granite, and gray granite. Most of the pink granite occurs in the area to the southwest of St. Cloud (Figure 81) . The rock is best described as having large pink crystals set in a finer grained black and white background. The minerals of the matrix occur in remarkably uniform sizes, and the pink crystals are sufficiently uniform in their distribution to give the stone a very at­ tractive appearance. It is marketed under such trade names as "Rockville Pink," "Cold Spring Pearl Pink," and "Original Minne­ sota Pink." "Rockville Pink" is exceptionally coarse-grained, the angular feldspar crystals being Y2 to % of an inch long. The granite consists of pale pink feldspar, quartz, and black mica, the comhned effect of which, on a hammered surface, is pinkish-gray. The red granite of the St. Cloud region is a medium- to coarse­ grained rock, with feldspar grains averaging about 14 of an inch in diameter. The chief minerals are feldspar and quartz, with minor amounts of black hornblende and biotite. Red or pink feldspar con­ stitutes about 75 per cent of the rock and gives it its red color, while the quartz occurs as coarse, glassy grains. The finished product may be called "Indian Red," "Rose Red," "}\lelrose Red," Ruby Red," or ":Mahogany Red." :Many of the quarries that were once promi­ nent are now inactive. Most of the rock was used for monumental purposes, but some architectural stone was fabricated also. The gray granite of the St. Cloud region occurs in the same general region as the red, and the two are more or less intimately associated. The gray stone is finer grained than the red, with the feldspar grains averaging about lis to %6 of an inch long. The minerals that may be easily recognized are gray feldspars, black hornblende or mica, and colorless quartz. In some regions quartz is more abundant than hornblende, but it is never very prominent. The stone owes its dark gray color to the gray feldspar and associated black minerals. The gray granite is marketed under such trade names as ":l\Iinne­ sota Dark Gray," "Pioneer Dark Gray," and "Reformatory Gray," the last name resulting from the fact that the great stone wall of the state reformatory at St. Cloud is built of this stone, and several large quarries were formerly active within the reformatory wall. The 176 MINNESOTA'S MINERAL RESOURCES term "crystal gray" is applied to a rock that has exceptionally large phenocrysts in a dark gray matrix. It differs from the coar e-grained "Rockville Pink" in that the rock has a greeni h-gray color and many of the feldspar grains are larger. Some of the rock contains large blue quartz grains, which add to its dark gray color. Various types of granite also crop out on the floor of the Minne- ota River Valley for a distance of 75 miles. -;\10 t of these outcrops have been exposed by the erosive action of the Glacial River Warren that once drained Glacial Lake Agassiz during the waning stages of the last glacier. In the region near Morton and Redwood Falls. sev­ eral mounds of granite gneiss stand from 75 to 100 feet above the level of the Minnesota River (Figure 84), and a number of quarries now market this rock (Figure 85) , a pink and black biotite granite gneiss with contorted laminations or bands. Quarries are also active in the .Minnesota Valley near l\lontevideo and Sacred Heart. The Montevideo stone is red, with some black bands of biotite that give it a gneissic structure, but the banding is not as irregular and contorted a in the Morton granite. Black knots Figure 84. Granite outcrops on the floor of the Minnesota River Valley south of Sacred Heart. 177 MINNESOTA'S ROCKS AND WATERS Figure 85. Granite quarry on floor of Minnesota Valley at Morton, Minnesota. (Photograph by Cold Spring Granite Co.) of biotite, quartz and aplite veins, and irregular pegmatite areas characterize much of the rock. Some of the stone is of uniform struc­ ture and grade, however. At the Sacred Heart quarries a pink gran­ ite of medium-grained texture is produced. At some places the joint­ ing of the rock is too closely spaced for large blocks of dimension 178 MINNESOTA'S MINERAL RESOURCES stone, but successful quarry operations are carried on by discarding the smaller quarried blocks. Most of the stone from this region is sold under the trade name "Sacred Heart Pink Granite." Near Ortonville and Odessa, at the upper end of the Minnesota Valley where the Minnesota River drains Big Stone Lake, quarry­ ing has been carried on for a great many years. The rock in this area is a red biotite granite that is somewhat gneissic, but the gneissic texture is not so pronounced as at Montevideo and at Morton. Another area of granite rock is located in a zone that extends northeastward from St. Cloud across Benton and Mille Lacs coun­ ties. Most of the rock is buried deeply under glacial drift, but here and there low, dome-shaped masses crop out at the surface. At a quarry operated by the Cold Spring Granite Company five miles south of Isle (Figure 82) , the joints and sheeting planes of the rock are at such distances that blocks as large as eleven feet on a side may be cut. The rock is a comparatively light gray granite composed of large, white feldspar crystals embedded in a matrix of colorless quartz and biotite. The larger of the feldspar grains range from 1;,1 to Yz of an inch in length. The quartz grains average no more than liJ.6 of an inch in diameter, and the biotite occurs in small flakes scattered between the other two minerals. This granite is similar in texture to the stone quarried at Rockville but differs in the color of the feldspar crystals. Cut finishes, such as sawed, axed, or hammer­ ed, leave the stone nearly white. Because of its uniform color and distant spacing of fractures, many types of structural material are fabricated from it.