RESOURCE GEOLOGY, 45(1), 11•`23, 1995 Structural Control of Ore Deposits in Boulder County, Colorado, U. S. A. Zhigang Xu* Abstract : Ore deposits within Boulder County, lying in the Front Range of Colorado, mainly formed during the Laramide orogeny and constitute one part of the Colorado Mineral Belt. The Front Range of Colorado is a Precambrian crystalline core. Tectonically, the Colorado Mineral Belt is located within the middle Precambrian Colorado Lineament. The conspicu- ous NE-trending belt with NW-trending major faults includes ancestral Laramide structures and has localized Laramide intrusives and ore deposits. Because the Front Range of Colorado occupies an unstable foreland, located between the stable cratonic region to the east and the mobile geosynclinal fold belt to the west, it must have the characteristic deformation of a transitional belt, which underwent oblique vertical tectonism. This is reflected by these major faults and most of the vein fissures having vertical offsets with a certain horizontal component. Laramide intrusives occurring within Boulder County belong to the silica-saturated monzonite suite and mainly formed during about 70-60 Ma. and 55-45 Ma. In addition, there may exist some post-Laramide intrusives as dikes related to tungsten mineralization. The ore deposits within Boulder County mainly occur as fissure veins and are located within northeasterly fissures and northwesterly major faults. As fissure veins, the ore shoots are chiefly controlled by the intersections of major vein fissures and by the character of the vein fissures (the relative movement of fault walls and their changes in strike and dip). The ages of intrusives and related mineralization and the character of different vein fissures show that this area underwent three stages of structural deformation, magmatism and corresponding mineralization. During the early Laramide stage, this area was acted on by an oblique vertical tectonism with E-W directed compression, which resulted in uplifting of the Front Range, rejuvenation of some Precambrian faults and intrusion of some stocks as well as lead-silver mineralization; the northwesterly major faults have huge throws with a certain amount of left-lateral movement. During the mid Laramide stage, this area was acted on by uplifiting tectonism with SW-NE directed compression, which led to continuous uplifting, intrusion of some stocks and dikes and was accompanied by fluorite, pyritic gold and gold telluride mineralization; most NE- or E-NE-trending vein fissures have left-lateral movement with some vertical movement. During the late Laramide stage, this area was still acted on by an oblique vertical tectonism with SW-NE directed compression, which resulted in intrusion of some dikes and accompanied by tungsten mineralization; most east-northeasterly tungsten vein fissures have left-lateral movement. Comparing the structural stress fields of three stages with reconstructed maps of the North American, Kula and Farallon plates during the Laramide and post-Laramide periods, it may be found that the early Laramide oblique vertical tectonism with E-W directed compression was probably derived from the eastward subduction of the Kula and Farallon plates and the mid-and late-Lammide oblique vertical tectonism with SW-NE directed compression was probably derived from the northeastward subduction of the Farallon plate. However, through reading the abundant geo- 1. Introduction logical literature, in addition to some field work, Since SPURR et al.(1908) first defined the Colo- the author feels that some fundamental geological rado Mineral Belt, many geological and mining problems that are very important for structural investigations have been done, especially LOVER- analysis on ore deposits have not been solved so ING and GODDARD (1950) and LOVERING and far. For instance, were the Rocky Mountains pro- TWETO (1953) have synthesized research pertain- duced by vertical tectonism or horizontal com- ing to structure, magmatism and ore deposits in pression? If horizontal compression, was it in an the Front Range and in the Tungsten district of E-W direction or in a NE-SW direction? Are the Boulder County. conspicuous NE-trending Colorado Mineral Belt and NW-trending major faults with an important Received on December 2, 1993, accepted on March 1, 1994 * Inst . Mineral Deposits, Chinese Acad. Geol. Sic., 26 impact on the localization of ore deposits, Baiwanzhuang Road, Beijing, 100037 PRC Laramide structures or Precambrian structures re- Keywords: Structural control, Ore deposits, Boulder County, activated in the Laramide orogeny? What kind of Laramide structure, Intrusives, Mineralization, Structural relative movement did these faults undergo? stress field 11 12 Zhigang Xu RESOURCE GEOLOGY: Fig. 1 Map showing some different opinions about the origin of the Laramide structure. This paper proposes the author's opinion on ized by the Laramide deformational style and his- structural analysis of ore deposits in Boulder tory, distinct from adjacent regions. County on the basis of earlier publications and on The Front Range, consisting mainly of Precam- his own field work (Fig. 1). brian rocks, was a positive area during most of the Paleozoic and Mesozoic Eras. It was submerged 2. Regional Structural Background during most of the late Cretaceous, and began to of the Colorado Mineral Belt ascend from the middle stage of the late Creta- Boulder County is located at the northeast end ceous (about 67.5 Ma.) until the Paleocene (LOV- of the Colorado Mineral Belt, which extends ERINGand GODDARD,1950; TWETO,1975). So northeastward from the San Juan Mountains to the Front Range is one of the first-order uplifts near the city of Boulder. The Colorado Mineral formed by the Laramide orogeny. Belt within the Front Range extends from Breck- As a regional geological entity, the Colorado enridge to Jamestown, about 100km long and 32 Mineral Belt was first recognized as tectonic in km wide (Fig. 2). origin and Laramide in age by SPURRet al. (1908). The Rocky Mountains of Colorado occupy, in They interpreted the NE-trending vein fissures the sense of structural deformation, an unstable and the NW-trending faults as complementary foreland which lies between the stable cratonic re- shear fractures produced by E-W compression gion of the Great Plain Province to the east and the that created the mountain ranges. Later, SPURR mobile block-faulted and formerly geosynclinal (1923) considered that the mineral belt was an ex- belt of the Basin and Range Province to the west, pression of an underlying "magma channel". and comprise a single tectonic province character- Since that time, geologists have felt that the min- 45(1), 1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A. 13 Fig. 2 Geologic map of the northern part of the Colorado Front Range (from LOVERINGand GODDARD,1950). The names of mining districts: 1-Breckenridge, 2-Montezuma, 3-Argentine, 4-Silver Plume, 5-Jones Pass, 6-Georgetown, 7-Empire, 8- Lawson-Dumomt, 9-Alice-Yankee Hill, 10-Central City-Idaho Springs, 11-North Gilpin County, 12-Eldora, 13-Caribou, 14-Ward, 15-Tungsten, 16-Magnolia, 17-Gold Hill, 18-Jamestown. 14 Zhigang Xu RESOURCE GEOLOGY: eral belt is fundamentally tectonic and magmatic. sistent for 20-40km and are spaced 3-6km apart, However, the origin of structure which devel- but their northern parts swing to the W-NW. oped during the Laramide orogeny in the Rocky Many of these faults were recognized earlier by Mountains has been the subject of debate. The TWETOand Siivms(1963) to have a Precambrian main issue, as pointed out by CONEY(1976), has ancestry. The areas between the major faults are been whether the basement-core uplifts were broken by many minor younger faults and vein caused by regional compression or vertical tec- fissures, most of which strike northeastward. tonic forces (Fig. 1) These NW-trending faults have localized Laramide intrusives and certain ores. These brec- 3. Precambrian Rocks and Structure cia reef faults show downthrows of SW sides, as The Front Range is a regionally approx. N- much as 300-400m. Because the unconformity trending Precambrian crystalline core (anticline). between the Precambrian rocks and the Paleozoic The anticline consists of the schists and gneisses -Mesozoic strata in the eastern edge of the Front of the Idaho Springs formation (pre-1,700 Ma.) Range dips outwards (southeastward), the down- and was greatly modified by extremely complex throw of the southwest sides of the breccia reef crenulation, cross folds and longitudinal folds and faults resulted in a "right-lateral displacement" of is intruded by large granitic batholiths of Boulder the unconformity lines on the geological map (i.e. Creek granodiorite (1,700 Ma., PETERMANet al., the erosion surface)(Fig. 2). Of these breccia reef 1968), Silver Plume granite [1,220-1,335 Ma. faults, the Maxwell and Hoosier reefs are most (MOENCH et al., 1962) or 1,400 Ma. (PETERMAN typical. et al., 1968)] and Pikes Peak granite (980-1,050 The NE-trending mineral belt within Boulder Ma., HEDGE, 1970)(Fig. 2). Besides these, there County expresses a general NE-trending vein fis- exist many Precambrian dikes of aplite, alaskite, sure zone from the Caribou-Eldora area to the pegmatite, and less common mafic dikes. Jamestown area (Figs. 2 and 3). The displace- The Idaho Springs formation mainly consists of ments on these faults appear to be small, less than quartz-biotite schists or gneisses, with minor 15m, as was noted by LOVERINGand TWETO quartzites and impure limestones, locally contain- (1953). ing scheelite-bearing calc-sillicate lenses. The Boulder Creek granodiorite occurs as stocks and 5. Laramide Intrusives small batholiths, chemically it is a calc-alkalic Laramide intrusives within the Front Range can rock with high ratios of K2O to-Na2O. The Pre- be classified into a group of stocks and laccoliths cambrian rocks may be the ancestral sources of and a second group of dikes, sills and plugs.