However, Through Reading the Abundant Geo- 1

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 conspicuous 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. The tungsten, lead, zinc, copper, silver, gold, molyb- radiometric ages of these intrusives within the denum and uranium in Laramide ore deposits, e.g. Front Range of Colorado show two age groups: an reworked Precambrian scheelite deposits may older 70-60 Ma. group and a younger 53-44 Ma. have been a source of tungsten in the economi- group. Besides, some stocks, dikes and even cally important Tertiary deposits (TWETO, 1960) . batholiths were emplaced during the post- Laramide tertiary (commonly Oligocene, 40-25 4. Laramide Structure Ma.). These intrusives may be an expression of a The most distinct Laramide structural feature is belt of underlying batholiths, as pointed out by the NE-trending mineral belt (and intrusions) ex- TWETO(1975). tending from Breckenridge to Jamestown. Other The Laramide igneous rocks of the Colorado distinct Laramide features are the NW-trending Mineral Belt can be divided into two petrographi- folds and faults developed in the crystalline rocks. cally, chemically, geographically and originally In the eastern edge of the range, particularly south distinct suites derived from different types of of Boulder, the echelon faults with downthrows source materials (BRADDOCK, 1969; SIMMONS on the west become the very distinct structures and HEDGE, 1978), as follows: that were called "breccia reefs". 1. A silica oversaturated granodiorite (quartz- These NW-trending breccia reef faults are per- bearing monzonite) suite, occurring over almost 45(1), 1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A. 15

Fig. 3 Geologic map of the Boulder County (from LOVERJNGand GODDARD, 1950). Main mines: 1-Caribou, 2-Illinois, 3- Conger, 4-Clyde, 5-Cold Spring, 6-Rogers I and II, 7-Slide, 8-Interocean, 9-Emancipation, 10-Poorman, 11-Logan, 12- Ingram, 13-Kekionga, 14-Mt. Lion-Keystone, 15-Lady Franclin 16-Golden Age, 17-Crand Central 18-John Jay, 19-New Rival Black Rose, 20-Smuggler, 21- Alice Burlington County belong to the silica-saturated monzonite the entire length of the Colorado Mineral Belt. suite. MUTSHLER, et al. (1988) called the above- These rocks belong to a calc-alkaline rock suite mentioned granodiorite and monzonite suites calc and have Sr contents less than 1,000 ppm, subpar- -alkaline and alkaline rocks respectively and con- allel REE patterns and initial 87Sr/86Sr ratios sidered that the former represents the partial melt- greater than 0.707. Their parent magmas may be ing of a lower crust plagioclase-pyroxene granu- derived from partial melting of a mixed source. lite assemblage caused by ponding of mantle-de- 2. A silica saturated monzonite suite, being re- rived alkaline rocks and volatiles in the lower stricted to the northeastern part of the Colorado crust, the latter represents the lithospheric mantle Mineral Belt, where few rocks of the granodiorite melting initiated by upwelling of asthenospheric suite occur. These rocks may be subdivided into mantle and metasomatic fluids related to vertical rock units: Alkali monzonites, mafic monzonites tectonism. LIPMAN (1980) pointed out that the and quartz syenites as well as their derived ultra- Laramide and Tertiary igneous rocks in the Colo- mafic rocks, and belong to an alkali-calcic rock rado sector become more alkalic to the east (i.e. suite and have Sr contents greater than 1,000 ppm, the silica saturated monzonite suite), indicating highly variable REE patterns and 87Sr/86Sr initial generation from progressively greater depth. ratios less than 0.706. The chemical and isotopic 6. Features of Mining Districts and differences serve to emphasize that the alkali Their Main Fissure Veins monzonites, mafic monzonites and quartz syenites cannot be related by fractional crystallization In the Colorado Mineral Belt of the Front Range from a common parent. there are eighteen mining districts, of which the Almost all Laramide intrusives within Boulder Eldora, Caribou, Ward, Tungsten, Magnolia, 16 Zhigang Xu RESOURCE GEOLOGY:

Gold Hill and Jamestown mining districts are located in Boulder County. These mining districts constitute a NE-trending Laramide porphyry and mineralization zone (Figs. 2 and 3).

1. The Caribou district is located in the northern end of the N-NE-trending Caribou-Empire anticline, and at the intersection of the NW-trending Maine-Cross Breccia Reef fault and the NE-trend- Fig.4 Geologic plan of level of the Illinois mine, Tung- sten district (from LOVERINGand TWETO, 1953). ing Berthoud Pass fault. The Caribou stock is a composite intrusive mass composed chiefly of monzonite with masses of titaniferous magnetite and bodies of ultramafic and gabbroic rocks.

In the district, there exist two sets of veins form- ing an interconnecting vein system containing lead-silver ore (See Fig. 8). The NE-trending No Name vein is the strongest and most persistent vein in the district, strikes N65•‹E and dips 55-65•‹

NW. Another set of veins strikes E-W and includes more than 10 veins, of which the Caribou vein is the most typical and important. Based on the fact that none of the easterly veins cut across the No Name vein and most of them abruptly stop Fig. 5 Geologic plan of Rogers mine, Tungsten where they reach the No Name vein, it can be in- district (from LOVERINGand TWETO,1953). ferred that the easterly veins formed as tension fractures in the wall of the right-lateral shear zone trending sections of branch fissures develop ore

of the No Name vein. shoots and the NE-trending sections are barren 2. The tungsten district extends in a narrow suggests the branch fissures underwent left-lateral northeasterly belt 15km long and 1.5-3km wide. movement which may be derived from the right-

Tertiary dikes include hornblende monzonite por- lateral movement of the Illinois vein (Fig.4). In the Tungsten district there also exist a few N- phyry, hornblende diorite porphyry, biotite monzonite porphyry and associated intrusion breccias. NE-or NE-trending veins with right-lateral move- The NW-trending breccia reef faults and the E- ment. For instance, the Rogers I vein right-later-

NE-trending fissure vein zone constitute the con- ally offset the NW-trending Rogers breccia reef spicuous structure in the district. Most veins were for 6 m and has a downthrrown northwestern wall. ferberite-mineralized and have preserved a record So the Rogers I vein fissure and the E-NE-trend- of repeated shearing-some prior to and some at the ing left-lateral Rogers II vein fissure may be the same time as the mineralization. Some gold-tellu- conjugate fractures that were probably produced ride veins are present in the eastern part of the dis- by NE-SW directed compression (Fig. 5). trict and some lead-silver-zinc ores occur along 3. The Gold Hill district occurs in the northern

the northern and southern borders of the belt. part of the Boulder Creek granodiorite batholith, Most tungsten vein fissures are E-NE-trending which was invaded by porphyry dikes ranging with vertical or steep dips, and underwent re- from diabase to alaskite in composition. The most outstanding Laramide structures are peated shearing. They are usually left-lateral shear with a certain vertical component of movement in the strong northwesterly Maxwell and Hoosier the early-and/or middle-stage, and are right-lateral breccia reefs. Besides these, there also exist sev- shear in the late stage. For example, the Illinois eral west-northwesterly breccia reefs. The north- vein follows a premineral left-lateral shear zone westerly breccia reefs have generally a reverse- with 30m displacement and has some secondary left-lateral displacement ranging from 15m on the tension fissures; however, the fact that the N-NE- Hoosier reef to 180m on the Maxwell reef. These 45(1), 1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A, 17

mineralization (See Fig. 8). Some veins such as

the Grand Republic, Western Slope and Mud veins have E-or E-NE-or W-NW-trends and left- lateral offsets (Fig. 7). So it may be inferred that the northeasterly vein fissures with right-lateral

movement and gold telluride mineralization and the easterly vein fissures with left-lateral movement and gold telluride mineralization should belong to the conjugate vein fissures that resulted Fig. 6 Geologic plan of the from the E-NE (about N60•‹E) directed compres- Emancipation mine, Gold Hill district (from LOVERING sive stress, much like the Emancipation mine. It and GODDARD, 1950). may also be inferred that the left-lateral movement of some NE-trending vein fissures with pyritic gold mineralization was probably produced

by elastic rebound after release of NEE-SWW directed compressive stress, but it acted for only a

short time; after that the area may have experi- menced a NEE-SWW directed compressive stress field (see Fig. 8). Of course, the actual situation may be more complicated, e.g. the Logan, Mud

and Teller Veins in the Logan mine (Fig.7). 4. The Ward district occupies the west-central

part of Boulder. County. There are six moderately extensive Laramide stocks of diorite and monzonite porphyry and a few small irregular masses of sodic andesite and diorite porphyry. The veins

near Ward mostly trend west-northwestward and dip north (usually steeper than 60•‹ and abruptly change to 15-30•‹) and coincide with dike trends Fig. 7 Sketch plan of the Grand Republic, Logan and and belong to the "breccia reef" fault system. Yellow Pine mines, Boulder County (from LOVERING Near Sunset there also exists a strong NW- and TWETO, 1953). trending fault (with a dip of about 80•‹NE) that two reefs were left-laterally displaced by the may belong to the Rogers breccia reef system. The Poorman reef. In this district there are more than fissure veins strike both NE and NW. The NE- 100 fissure veins. Of these veins, about 90% of the trending vein fissures have a right-lateral-reverse veins occur between the Maxwell and Hoosier movement, and the NW-trending vein fissures breccia reefs and almost all important veins are in have a left-lateral movement. So they may belong the immediate vacinity of the breccia reefs. Most to the conjugate fractures that were produced by of the fissure veins strike N30-45•‹E and dip an E-W directed compressive stress (see Fig. 8). steeply. 5. The Jamestown district is located in central The veins have different strikes and dips and Boulder County. Laramide intrusives comprise underwent multiplemovements. In general, most the James Creek granodiorite stock and Porphyry fissures trending N 30-45•‹E display offsets along Mt. sodic granite-quartz monzonite stock as well normal or reverse-right-lateral faults, e.g.the as a few dikes. Interocean vein, Emancipatin vein (Fig. 6) and the Conspicuous structures include the northwest- early stage of the Ingram and Slide veins with erly breccia reefs and the northeasterly vein fissures. In fact, the Jamestown district occupies the gold-telluride mineralization (Fig. 3); but some of the veins, such as the Slide vein, again underwent area between the Standard and Hoosier reefs, a normal-left-lateral movement with pyritic gold whereas the persistent Maxwell reef passes 18 Zhigang Xu RESOURCE GEOLOGY:

through the district's center and is also cut by the granodiorite stock. In the Jamestown district there are four ore types: lead-silver, fluorspar, pyritic gold and gold telluride (in the order of decreasing age). The lead-silver mineralization is associated with fluorspar bodies and mainly fills in the northwesterly fissures. Pyritic gold and gold telluride deposits occur in NE-trending fissures and constitute a rough zonal distribution taking the sodic granite stock as a center (Fig. 3). In the Golden age mine, the Golden age pyritic gold vein is cut by the Sen- tinel telluride vein. Based on the fact that the fluorspar occurs as primary minerals in a late phase of the sodic granite stock and that the fluorspar-bearing veins and breccia zone are cut by granitic dikes (in the Emmett mine), it may be considered that the fluorspar deposits are probably related to the sodic granite porphyry in genesis. The NE-trending fissure and pyritic gold and gold telluride are considered to be younger than the NW-trending fissures and fluorspar on the basis of minor galena, sphal- erite and telluride minerals filling fractures in purple fluorite in the fluorspar deposits (KELLY and GODDARD,1969). Some lead-silver ores are believed to be close to fluorspar and some to telluride in age. The NE-trending persistent vein fissures in both eastern and western sides of the granodiorite stock have a right-lateral shear movement and the stock has an elongate shape in S-N direction, which suggests that these vein fissures may be produced by an E-W directed compressive

stress. However, east-northeasterly vein fissures in the northern and southern margins of the granodiorite stock are reverse faults and have low-grade

pyritic gold, which may be related to upward- southward and upward-northward forces accom-

paning the intrusion of the granodiorite stock. As for the Smuggler vein north of Jamestown which

strikes N and has right-lateral movement, it may be related to a SW-NE directed compressive stress

(see Fig. 8). 6. The Magnolia district lies about 9km west of Boulder. The Livingston breccia reef fault trending N25•‹W passes through the district. All the ore deposits in the district are chiefly of the gold-tellu-

ride type, but some tungsten ore has been mined. In the Kekionga-Magnolia mine, fine-grained 45(1), 1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A. 19 ferberite veins cut tellurides. Almost all veins lie ments and are controlled by certain structural in a zone about 2.4km long and 0.8km wide. stresses. Some veins constitute complex veins be- Based on this fact, in addition to swinging of the cause of spliting or convergence of veins or di- Livingston and Hoosier reefs, it may be inferred vergingbranch veins. that the veins may reflect the presence of a deep Ore shootsare veryimportant sites of mine pro- underlying northeasterly cross channel with left- duction and were mainly controlled by the follow- lateral shear movement which may be related to ing structural factors. the left-lateral shear movement of the Tungsten 7.1 Ore shoots controlled by the intersection Mineral Belt, although the main mineralization of vein fissures (tungsten) was later than gold telluride mineral- This kind of ore shootis the most common.For ization (see Fig. 8). instance,in the Jamestown,about 75% of ore out- put has come fromvein junctions.Most vein inter- 7. Structural Control of Ore Deposits sections belong to different directed fractures and Ore Shoots formed during different structuralperiods. Some From the above statements,it can be inferred vein intersectionsformed during different stages that the NW-trendingbreccia reef faults have had of the same structuralperiod. an important impact on localizationof Laramide Of course,some vein intersectionsmay belong and late-Laramide intrusives and ore deposits. to conjugate fractures formed during the same The Cariboustock may lie near the intersectionof stage and same structuralstress field, such as the the NW-trendingMaine-Cross breccia reef fault Emancipationmine (Fig. 6), and the Rogersmine and the NE-trending Berthoud Pass fault. The (Fig. 5). Some vein intersections belong to the stocks in the Ward district intrude where the masterfault and its branchfaults formed underthe Livingstonand Rogers breccia reefs swing from same structuralstress field, such as the No Name the NW-trendto W-NW-trend.In the Gold Hill (master)vein with the Caribou(branch) vein (Fig. district, about 90% of veins occur between the 8) and Mud vein with its branch vein. Maxwelland Hoosier brecciareefs and almost all 7. 2 Ore shoots controlled by the character importantveins are near the reef faults. of vein fissures Ore depositsin BoulderCounty mainly occur as This governing factor includes the relative vein fissures..The veins were formed by infilling movement of fault walls and the achanges of of ore-bearingsolutions into fracturesand chiefly courses and dips of vein fissures.Very few frac- occur in the followinggeologic settings. turesare simpleplanes. Theyusually have irregu- 1. Veinsfollowing breccia reef faults.This geo- lar outline along both strike and dip. The relative logic settingtakes the YellowPine vein in Gold movementof fissure walls may produce second- Hill districtas the typical example.It follows the ary micro-tensionalstructures in suitableplaces. southwestern edge of the Hoosier breccia reef Althoughthis kind of micro-tensionalstructure is fault for about200m (Fig.,7). Anotherexample is very small (generallyspeaking, the displacement the Standardpyritic gold vein in Jamestowndis- of 1 m along the fault only results in a tension- trict whichfollows the Standardbreccia reef fault. distanceof 0.1-0.2m or less betweentwo walls),it 2. Veins following dikes of Precambrianpeg- may express the micro-tension'several tens of matite (or aplite)or Laramideintrusive dikes. This meters in length and width along the course and geologicsetting can be foundin manyplaces, e.g. dip of the fault. For a normal fault, secondarymi- the Illinois vein in the Tungstendistrict (Fig. 4), cro-tensionsfor ore shootsoccur where the dip of and the Gold Medal vein in the Poormanmine of the fault becomes steeper;but for a reverse fault, the Gold Hill district. secondarymicro-tensions occur where the dip of 3. Veins following premineral fractures. This the fault becomes more gentle. Similarly, for a geologic settingis most commonin ore deposits left-lateralfault ore shoots occur where the vein of Boulder County. Most of these fractures are course swings to the left; for a right-lateralfault Precambrian and were rejuvenated in the ore shoot occur where the vein course swings to Laramiderperiod, usually underwentmultimove- the right. Howevermost faults have both vertical 20 Zhigang Xu RESOURCE GEOLOGY:

and horizontal movement(i.e. oblique-slip move- right-lateral and NW-trending left-lateral faults ment), so many ore shoots have pitching shapes with steep dip, the E-W-trending tensional faults on longitudinal sections. with steep dip and the N-S-trending thrust faults In fact, some larger ore shoots were controlled with gentle dip (see Fig. 1). Although the NW- by several factors. The Slide vein has the deepest trending major faults in this area have certain left- ore shoot, extending from the surface to a depth of lateral horizontal movement, the NE-trending 300m. The localization of this ore shoot is related vein fissures such as those in the Tungsten district to the junction of two veins and the left-lateral have left-lateral movement with less than 15m movement of the late stage would tend to open the displacement, which is contrary to the theoretical N50•‹E part of the vein and to tighten the N20•‹E results.

part of the vein (see Fig. 8). Similarly, NE-SW directed compressive stress, as pointed out by CONEY(1976), may explain the 8. Analysis of Structural Stress Field left-lateral movement of the northeasterly vein fis- The analysis of structural stress field is very im- sures, but it cannot explain the left-lateral move- portant for inferring the structural control of ore ment of the NW-trending major faults. deposits, but it is also difficult because some fun- From the two groups of Laramide intrusives in damental problems of regional structure have not age and the character and sequences of the relative been resolved, the vein fissures usually underwent movement and mineralization of vein fissures multimovement and multimineralization, and mentioned above, it may be inferred that the intru- their relations are obscure in many places. In ad- sive activity, mineralization and related structural dition, the change of boundary conditions also in- control (stress field) may be broken into three creases the difficulty in the analysis of the struc- stages (Figs. 1 and 8). tural stress field. So the analysis of the structural The early stage comprised the period of about stress field is only discussed preliminarily. As 70-60 Ma. and may be called the early Laramide mentioned above, these conspicuous NW-trend- stage. During this stage the area was under the af- ing faults are Precambrian, and rejuvenated in the fect of an oblique vertical tectonism with E-W di- Laramide orogeny. The Laramide structural stress rected compression, which resulted in rejuvena- field and its products was influenced by these Pre- tion of major northwesterly Precambrian faults cambrian faults, evidenced by some Laramide with larger vertical throw and certain left-lateral faults utilizing and following Precambrian faults. movement. It also resulted in intrusion of the Because this area is located at the margin of the Audubon-Albion, Caribou-Bryan Mt. (Eldora) North America platform and occupies an unstable and James Creek stocks with S-N directed elon- foreland which lies between the stable cratonic re- gated shapes and the related lead-silver mineral- gion with vertical tectonism and the mobile geo- ization in the Caribou district and in the southern synclinal folded belt with horizontal tectonism, it and northern margins of the James Creek stock as underwent the tectonism and deformation charac- well as the Yellow Pine mine which follows the teristic a transitional belt. The major uplifts and Hoosier breccia reef fault. In the Caribou district basins of the Rocky Mountains with huge throw the northeasterly No Name (master) vein with may be produced by dominantly vertical move- right-lateral movement appears to be the product ment associated with horizontal compression, i.e. of E-W directed compression. by an oblique vertical tectonism. As a result, most During the middle stage of about 55-45 Ma., of the conspicuous NW-trending major faults and which may be called the mid Laramide stage, this NE-trending vein fissures have steep dips and area was controlled by oblique vertical tectonism, large vertical offsets with small horizontal ones. with SW-NE directed compression, which led to For instance, the Maxwell and Hoosier breccia reactivation of the northeastely fissures and the reef faults have vertical displacements of 300-400 intrusion of the stocks in the Ward district, the m and horizontal displacements of 60-120m. Porphyry Mt. stock in the Jamestown district, as Theoretically, an .E-W directed horizontal com- well as certain dikes. These northeasterly or east- pressive stress may produce the NE-trending northeasterly fissures usually display left-lateral 45(1), 1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A. 21 offsets and are infilled by pyritic gold or gold telluride ores. It should be pointed out that the NE-trending fissure zones in the Tungsten district, except a few fissures opened and were infilled by pyritic gold and gold telluride ores, only happened the left-lateral shear movement, which resulted in the Mag- nolia northeasterly secondary left-lateral shear zone as well as accompaning W-NW-trending tertiary pyritic gold and gold telluride vein fissure with left-lateral movement. In the Jamestown district the pattern of relative movements of vein fissures in different places shows that the local stress field was very complex: some fissures may be related to SW-NE directed compression, some related to the eastward compression, and some related to westward compression (Fig. 3). These eastward and westward compressions are inferred to belong to the secondary E-W directed tensional stresses which were derived from the SW-NE directed compressive stress when it interacted with the James Creek granodiorite stock. The late stage refers to the intrusion of hornblende monzonite porphyry and biotite monzonite porphyry and tungsten mineralization in the Tung- sten district and may belong to the late-Laramide stage. The left-lateral movement of the E-NE- trending vein fissures and the right-lateral movement of the N-NW-trending vein fissures indicate that this area underwent the SW-NE directed compression. The regional stress fields that controlled the structure-mineralization of three stages in the north-central Front Range of Colorado may have resulted from the movement of the North America plate relative to the Kula and Farallon plates (Fig. 9). The E-W directed compression of the early stage may have resulted from the eastward subduction of the Kula and Farallon plates, while the SW-NE directed compression of the middle and late stages appears to have resulted from the northeastward subduction of the Farallon plate Fig. 9 Plate reconstruction during Laramide orogeny (CONEY, 1976). based on relative motions(North America assumed 9. Conclusion fixed)(From CONEY, 1976). Heavy barbed lines=subduction zones; light barbed lines=low-angle thrust Ore deposits within Boulder County occur in fault belts; shaded-dashed areas=volcano-plutonic the Colorado Mineral Belt and mainly formed belts of arc-affinity. Vectors in cm/yr calculated with respect to North America for period 65 to 55 M.Y.. during the Laramide orogeny. The Front Range of Colorado is a Precambrian crystalline core. The 22 Zhigang Xu RESOURCE GEOLOGY:

Precambrianrocks may represent, the source of recting my initial literature and field work on this gold, silver and tungstenmineralization remobi- topic. I also benefitted from Edwin E. LARSON'S lizedduring the Laramideorogeny. The conspicu- research into Cenozoic magmatism in Colorado, ous northeasterly lineament and northwesterly Larry WARNER'sresearch into Colorado's regional major faults constitute precursors of Laramide and structural evolution, and LOVERING and structuresand had an importantimpact: on the lo- GODDARD's structural descriptions of ore depos- calizationof Laramideintrusives and oredeposits. its. In addition, I wish to thank graduate students Ore depositswithin the BoulderCounty mainly Bruce A. GELLERand Avrom E. HOWARDfor as- occur as vein fissures. Genetically,these ore de- sistance in the field, and anonymous reviewers posits weremainly related to the rocksof the silica from the Society of Resource Geology. -saturated monzonite suite with more alkaline contents and were controlledby the Precambrian Reference northwesterlymajor faults and northeasterlyfrac- Boos, C. M. and Boos, M. F.(1957): Tectonics of the eastern tures rejuvenatedin the Laramide orogeny. The flank and foothills of the Front Range , Colorado, Am. ore shootswere mainly controlled by intersections Assoc. Petroleum Geologists Bull., 41, 2603•`2676 of fissureveins and by characteristicsof vein fis- BRADDOCK, W. A.(1969): Geology of the Empire quadrangle, Grand, Gilpin and Clear Creek Counties, Colorado, U . S. sures.The latterincludes the relative movement of Geol. Survey Prof. Paper 616, 56p. fault walls and changesof strikeand dip. BURBANK, W. S., (1933): Relation of Paleozoic and Mesozoic The ages of intrusives,mineralization and rela- sedimentation to Cretaceous-tertiary igneous activity and tive movementof vein fissures indicate that the, the development of tectonic features in Colorado, p.277•` area has undergoneat least three stages of mag 301. In Ore Deposits of the Western States (Lindgren Vol- ume), Am. Inst. Mining Metall. Engineers matism and related mineralization,which repre- , 797p. sent interactionsbetween the North Americaplate CONEY, P. J. (1976): Plate tectonics and the Laramide orogeny . and Kula and Farallon plates. During the early In Tectonics and Mineral Resources of Southwestern North Americal. New Mexico Geol. Soc. Special Publica- Laramidestage (about70-60 Ma .) the area started tion, No. 6, 5•`10. to uplift, under the action of an oblique vertical GODDARD, E. N. (1940): Preliminary report on the Gold Hill

tectonism with E-W directed compression that mining district, Boulder County, Colorado. Colorado Sci . was derived from the eastwardsubduction of the Soc. Proc., 14, No. 4, 103•`139. Kula and Farallonplates; the northwesterlymajor HEDGE, C. E. (1970): Whole-rock Rb-Sr age of the Pikes Peak faults are markedby hugethrows with certainleft- batholith, Colorado. U.S. Geol. Survey, Prof. Paper 700- B, 86•`89. lateral movement.During the late Laramidestage KELLY, W. C. and GODDARD, E. N. (1969): Telluride ores of

(about55-45 Ma.), becausethe Farallonplate sub- Boulder County, Colorado. Geol. Soc. Am. Mem. 109 , ductednortheastward beneath the North America 237p. plate, the area was acted upon by upwellingtec- LIPMAN, P. W. (1980):Cenozoic volcanism in the western tonism with SW-NE directedcompression; most United States:Implication for continental tectonics . In Continental Tectonics, National Academy of Sciences of the NE-orE-NE-trending vein fissureshad left- , lateral movementaccompanied by verticalmove- Washington, D. C., 161•`174. ment, and most of the NW-trendingvein fissures LOVERING, T. S. (1933): The structural relations of the porphy- ries and metalliferous deposits of the northeastern part of had a right-lateralmovement. The post-Laramide the Colorado mineral belt. In Ore Deposits of the Western structuralmagmatism is just an inferencebased on States (Lindgren Volume). Am. Inst. Mining metall . Engi- the fact that someigneous dikes and tungstenmin- neers, 797p. eralization were later than the late Laramide LOVERING, T. S. and GODDARD, A. N. (1950): Geology and ore intrusivesand pyritic gold and gold telluridemin- deposits of the Front Range, Colorado. U. S. Geol . Survey eralization.The left-lateralmovement of most NE Prof. Paper 223, 312p. or E-NE-trendingtungsten vein fissures suggests LOVERING, T. S. and TWETO, O. (1953): Geology and ore de- that the area was still under an oblique vertical posits of the Boulder County Tungsten district, Colorado. U. S. Geol. Survey Prof. Paper 245, 199p . upliftingwith SW-NEdirected compression . MOENCH, R. H., HARRISON, J . E. and Suss, P. K. (1962): Pre- cambrian folding in the Central City-Idaho Springs area Acknowledgements:I wish to thank Professors , W. W. ATKINSON,Jr., and Jefferey KURTZfor di- Front Range, Colorado. Geol. Soc. Am . Bull., 73, 35•`58. 45(1),1995 Structural control of ore deposits in Boulder County, Colorado, U. S. A. 23

PETERMAN, P. K., HEDGE, C. E. and BRADDOCK, W. A. (1968): Mountains. Geol. Soc. Am. Memoir, 144, 1•`44. Age of precambrian events in the northeastern Front TWETO, O. and SIMS, P. K. (1963): Precambrian ancestry of the

Range, Colorado. Jour. Geophys. Research, 73, No.6, Colorado Mineral Belt. Geol. Soc. Am. Bull., 74, 991•`

2277•`2296. 1014.

SIMMONS, E. and HEDGE, C. E. (1978): Minor-element and Sr- WARNER, L. A. (1956): Tectonics of the Colorado Front isotope geochemistry of Tertiary stocks, Colorado Min- Range. In Geological Record. Rocky Mt. Soc. Am. Assoc.

eral Belt. Contri. Min. Petrol., 67, 379•`396. Petrol., Geol., 173p.

SPURR, J. E., GARREY, G. H. and BALL, S. H. (1908): Economic WARNER, L. A.(1978): The Colorado lineament: a middle Pre- cambrian wrench fault system. Geol. Soc. Am. Bull., 89, geology of the georgetowm quadrangle, Colorado. U. S. Geol. Survey Prof. Paper 63, 422p. 161•`171.

SPURR, J. E. (1923): The ore magmas. New York, McGraw- WOODWARD, L. A. (1976): Laramide deformation of Rocky

Hill Book Co., 915p(quoted from TWETO and SIMS, 1963). Mountain foreland: Geochemistry and mechanics. In Tec- TWETO, O. (1960): Scheelite in the Precambrian gneisses of tonics and Mineral Resources of Southwestern North

Colorado. Econ. Geol., 55, 1406•`1428. America. New Mexico Geol. Soc. Spec. Publ. No.6, 11•`

TWETO, O. (1975): Laramide orogeny in the southern Rocky 17

米国コロラド州ボウルダー郡における鉱床の構造規制

徐志剛

要旨:Boulder郡に分布する鉱床は主としてLazamide造山貫入岩と関連鉱化作用の年代や鉱脈の性質から,本地域運動に関連して生成し,Colorado Mineral Beltの一部を構はLaramide造山運動に関連して構造変形作用,火成活動成している.Front Range of Coloradoは東方の安定地塊とおよび鉱化作用の一連のステージを3回(初期,中期およ西方の地向斜褶曲帯に挟まれた地域を占めているため, び後期)受けていることが示された.3回のステージにお漸移帯特有の変形作用を受けているものと考えられる. ける応力場をLaramideおよび後Laramide期におけるプレートの挙動と対照したところ本地域ではLaramide造山運動に関連してモンゾナイトが ,Laramide変動初期のE-W 70-60Maと55-45Maに貫入した.これに加えて,タング方向の圧縮場テクトニクスはKulaおよびFarallonプレートステン鉱化作用を伴った後造山運動の岩脈が見られる. の東方への沈み込みに起因したものであり,Laramide変鉱床は鉱脈型で,富鉱部は主な鉱脈割れ目が交差する部動中期および後期のSW-NE方向の圧縮場テクトニクスは分に規制されており,また鉱脈割れ目の性質にも規制さ Farallonプレートの北東方向への沈み込によるものであるれている. 事が明らかになった.