Historically, Colorado has experienced industrial mineral development with a surprising variety of materials including: gypsum, aggregates, silica, clay, limestone, cement, crushed stone, nahcolite (sodium carbonate), vermiculite, industrial gasses (helium, CO2), dimension & decorative stone, silica (frac sand) and so on.
Sand, Gravel, and Quarry Aggregates
- construction fill
- asphaltic mix/concrete
- road base
- other concrete and asphalt products.
Other uses of aggregate include rip-rap, plaster and gunite sands, snow and ice control, filtration systems, railroad ballast, hydraulic fracturing, and roofing granules. Aggregates are generally defined as granular materials that are: 1.) used with a cementing medium to form concrete or mortar or 2.) used as a construction material, meeting the requirements of road, paving, or other construction applications (modified from ASTM C125-18 and D8-18, 2018).
Figure 1 – Left: sand and gravel stream deposits along the Dolores River in western Colorado (M. O’Keeffe). Right: crushed rock quarry in granite, west of Denver, Colorado (M. Morgan).
Sand, gravel, and crushed rock are all quarried for use as aggregate in Colorado. Crushed rock can include igneous, sedimentary, and metamorphic rocks. This natural rock material must be blasted and crushed to the desired aggregate size. Igneous rocks form from molten rock or magma and include intrusive rocks (magmas that cooled at depth) and extrusive rocks (rocks produced by eruptions onto the earth’s surface). Igneous rocks that are typically quarried for aggregate include granite and basalt (Figure 2). Sedimentary rocks are formed by deposits of eroded igneous and metamorphic rocks as well as material produced by living organisms. Limestone, sandstone, and other sedimentary rock types are mined in Colorado for aggregate (Figure 3). Limestone is primarily mined and crushed for use as concrete aggregate while well-indurated sandstones (difficult to break with a hammer) can also be a good source for crushed stone. Metamorphic rocks include rocks that were subsequently transformed after being exposed to heat and pressure. Metamorphic rock types that are typically mined for aggregate include gneiss and schist (Figure 4). Several quarries in the Front Range mine gneiss and provide crushed rock to the Denver metropolitan area.
Figure 3 – Left: Pikes Peak granite near Colorado Springs, CO (M. O’Keeffe). Right: Basalt near Craig, CO (P. Barkmann).
Figure 4 – Limestone rock quarry in Colorado (J. Keller).
Figure 5 – Left: Outcrop of metamorphic rocks (gneiss) near Breckenridge, CO (M. O’Keeffe). Right: Close up of gneiss near Nederland, CO (M. O’Keeffe).
Sand and gravel deposits are typically unconsolidated or weakly cemented sedimentary deposits formed by several different natural processes including:
- fluvial (streams, also known as alluvial deposits)
- lacustrine (lakes)
- marine (oceans)
- glacial (glaciers)
- eolian (wind) processes.
A general diagram showing the relationships between aggregates and landforms is shown in Figure 6. Many alluvial deposits occur along modern and older stream drainages throughout the state and are typically quarried for aggregate. Common landforms associated with alluvial sand and gravel deposits include floodplain and low terraces along modern day streams, high terraces that are located tens of feet or greater above and along these streams, upland gravel deposits, and alluvial fans (Figure 6). Gravel and boulders from alluvium can also be crushed to obtain the desired size fraction of material. In the aggregate industry, fine aggregate is less than 9.5 millimeters (mm) (3/8-inch), most of which is less than 4.75 mm (3/16-inch) but greater than 0.075 mm. Coarse aggregate is predominantly greater than 4.75 mm.
Most sand and gravel quarried in the state occurs within the floodplain and underlies low terraces of modern streams that may contain less weathered aggregates (Figure 6). Terraces are remnants of former floodplains that formed when a stream was at a higher elevation prior to down cutting and forming a new lower floodplain (Figure 7). Other deposits include upland gravels located on interfluves (a region between the valleys) between drainages and alluvial fans which are fan-shaped wedges of sediment that form where a stream emerges onto an open plain from an area of higher elevation (Figure 8). Upland gravel deposits are associated with ancient stream courses and may or may not occur near modern streams and their tributaries. Both upland gravels and alluvial fans can be quarried for aggregate but upland gravel deposits typically are more weathered and may not be suitable for specific uses (Figure 7). Eolian sand deposits cover a large portion of eastern Colorado and are dominantly comprised of sand and silt (particles smaller than 0.031 mm) (Figure 9). Glacial sand and gravel deposits occur in former glaciated areas at higher elevations in the Rocky Mountains.
Figure 6 – General diagram showing the relationships between aggregates and landforms within and along the margin of the Colorado Front Range (Schwochow and Others,1974). Lowland landforms include valley fill (V), flood plain (F), and terraces (T1- youngest; T3 – oldest). Other landforms are upland gravels (U), alluvial fans (A), and wind-deposited sand dunes (E). Potential quarry (rock) aggregates are also shown and include fine-grained intrusive igneous rocks (Q1), fine-grained extrusive rocks (Q2), and large areas of coarse-grained igneous and metamorphic rocks (Q3).
Figure 7 – Upper left: Shaded relief map of the South Platte River and floodplain near Milliken, Colorado. Upper middle: South Platte River and floodplain near Milliken, Colorado. Upper right: Surficial geologic map of previous image showing the location of modern-day floodplain deposits and old alluvial terrace-forming deposits. Middle left: Glacio-fluvial (alluvial) deposits in Garfield County (M. Morgan). Middle right: Old alluvial terrace-forming deposits in Rio Blanco County (J. White). Lower left: South Platte River alluvial sand in the modern-day floodplain, Morgan County (M. O’Keeffe). Lower right: Photomicrograph of alluvial sand from the picture in the lower left (scale bar: 0.5 millimeters) (M. O’Keeffe).
Figure 8 –Left: Shaded-relief map of an alluvial fan northwest of Colorado Springs, Colorado. Middle: Satellite image of the same area. Right: Surficial geologic map of previous image showing the location of an old alluvial fan, young alluvial fan, and alluvial deposits (Geology from Cascade quadrangle geologic map).
Figure 9 – Left: Satellite imagery of the North Park sand dunes in Jackson County (NAIP, 2009). Middle: Eolian sand deposits in Morgan County. Right: Photomicrograph of sand collected at this location (scale = 0.5 millimeters) (K. Horkley).
Many factors determine what constitutes a viable sand, gravel, or crushed rock aggregate resource including:
- material composition, size, size distribution, color, shape, texture, and weight
- vertical and horizontal extent of the deposit
- overburden thickness
- material strength, durability, soundness, porosity, water adsorption, reactivity, specific gravity
- the presence of fines, sulfates, calcium carbonate, and other minerals
- environmental and mine permitting
- community relationships
- local supply and demand
- distance and transportation to the source
- available land for lease/purchase.
Although a natural aggregate deposit may meet many of these factors, developing these resources can be limited by issues associated with one or more of these elements. Also, one sand and gravel deposit may meet construction specifications for one type of application but may not meet the requirements for another. For example, sandstones and other sedimentary rock types may not meet engineering specifications for use in asphalt and concrete but could be used for road base or fill.
Colorado quarry operators produced an estimated 51.6 million tons of aggregate (sand, gravel, and crushed stone) in 2016. The estimated production between 1994 and 2016 is shown in Figure 10. In 2016, estimated sand and gravel production totaled 36.7 million tons and crushed stone production was 14.9 million tons. The Colorado Division of Reclamation, Mining, and Safety (DRMS) regulates mining in Colorado and manages a database that lists over 1,000 active permits for sand, gravel, aggregate, and aggregate-related quarries in Colorado.
Figure 10 – Sand and gravel (left) and crushed rock (right) production in Colorado, 1994-2016 (from O’Keeffe, 2018).
Colorado uses a large amount of aggregate to build and maintain needed infrastructure. The cost of aggregate to the end user is highly dependent on the cost of transporting aggregate. Locating quarries close to population centers helps lower overall costs. However, residential and commercial development near an aggregate source can make permitting a new or expanding mine a challenge.
The demand for aggregate is regional but land-use planning and permitting for aggregate supplies is done on a local level. Balancing the need for readily available and reasonably priced aggregate with other regional, social, and environmental priorities is not an easy task for local governments. A 1973 state law recognized the need to balance competing interests in urban areas, counties, with a population of least 65,000 residents, must develop a master plan for the extraction of industrial mineral deposits. The intent of this state law is to allow for the extraction of aggregate, and other commercial mineral deposits, while protecting the environment and public safety.
To help local Front Range governments identify potential sources of sand, gravel and quarry aggregates, the Colorado Geological Survey (CGS) created “Special Publication 5A and 5B, Sand Gravel and Quarry Aggregate Resources, Colorado Front Range Counties” (Schwochow and others, 1974). Digital versions of the aggregate resource maps can be found in “OF‐00‐09 Atlas of Sand, Gravel, & Quarry Aggregate Resources, Colorado Front Range Open File Report” (Cappa and others, 2000). These maps were published in an online interactive map server in 2018 (http://coloradogeologicalsurvey.org/).
Projected population growth in other counties throughout Colorado have prompted local governments in these areas to create mineral extraction plans in conjunction with future land use planning. Mineral extraction plans may prevent quality aggregate resource areas from being lost to other land uses.
Colorado Geological Survey Interactive Maps
Although the state has been mapped on a state-wide basis at a low resolution (small scale = generalized maps of larger regions), much of Colorado has not been mapped at a higher resolution (larger scales = detailed maps of smaller regions). Although much of Colorado’s geology is mapped at a state-wide lower resolution, it has become imperative to map with increasing detail that can support geologic investigations on a county or city-wide scale. Of the over 1,900 7.5-minute quadrangles in the state, more than 1,200 have not been mapped at the larger 1:24,000 scale. Under the joint state and federal STATEMAP Program, the Colorado Geological Survey continues to map areas of Colorado (1:24,000 scale) at larger scales. These detailed maps provide high-resolution (1 inch = 2000 feet on the ground) mapping of the surficial geology including potential aggregate resources such as alluvial sand and gravel deposits, alluvial fans, glacial/eolian deposits, and other igneous, metamorphic, and sedimentary rock formations. Most of these maps can be downloaded from the CGS bookstore or the USGS National Geologic Map Database. An update of the current status of geologic mapping is available on the CGS website.
- USGS national geological database – https://ngmdb.usgs.gov/ngmdb/ngmdb_home.html
Sand, Gravel, and Quarry Aggregates
In order to facilitate planning for urban growth, the CGS and USGS mapped the Front Range Urban Corridor’s sand and gravel resources in the 1970s. Those maps are summarized in “Colorado Geological Survey Atlas of Sand, Gravel and Quarry Aggregate Resources, Colorado Front Range Counties” published in 1974 (Schwochow and others, 1974). Several Front Range counties use these maps to develop mineral extraction plans. These maps were republished as an online interactive map which includes additional studies conducted by the CGS in other counties. The 1974 study included estimates of landform types, resource ratings, as well as boring locations with estimated thicknesses. The current interactive map includes these landform types, resource ratings, and the boring location data and can be accessed by the public on the CGS website (http://coloradogeologicalsurvey.org/).
The CGS uses geologic maps in many ways including the construction of mineral derivative maps. Mineral derivative maps are generalizations of detailed geological information that are used to assist with evaluating complex geological information with regards to general resource types including aggregates. Geological formations are evaluated for their potential as aggregate resources, as well as other types of resources, at a 1:24,000 scale. These maps and descriptions are included on a CGS interactive online map available on the CGS website (http://coloradogeologicalsurvey.org/mineral-resources/minerals-commodities/).
ON-007 MINERALS Landing Map https://cgsarcimage.mines.edu/ON-007 — This GIS map has not yet been deployed, but will include any/all the available GIS information that we have pertaining to Mineral Resources in Colorado.
The following maps are GIS data sets of specific topics:
ON-007.01 Aggregate Resources of Colorado —
ON-007.02 Colorado Historic Coal Mines —
ON-007.03 Mineral Resource Potential Derivative Map —
ON-007.04 Sand and Gravel Geology —
ON-007.05 StoryMap: Colorado Aggregate Resources – Geology and Industry Overview — Created by CGS Minerals geologist, Mike O’Keeffe this ESRI “StoryMap” explores this particular mineral resource around the state.