U.S. Department of the Interior

U.S. Geological Survey

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1729

Prepared in cooperation with the New Mexico Environment Department

Questa Baseline and Premining Ground-Water Quality 

Investigation 18. Characterization of Brittle Structures in the 

Questa Caldera and Their Potential Influence on Bedrock 

Ground-Water Flow, Red River Valley, New Mexico

Prepared in cooperation with the New Mexico Environment Department

Questa Baseline and Premining Ground-Water Quality 

Investigation 18. Characterization of Brittle Structures in the 

Questa Caldera and Their Potential Influence on Bedrock 

Ground-Water Flow, Red River Valley, New Mexico
Questa Baseline and Premining 

Ground-Water Quality Investigation 18. 

Characterization of Brittle Structures in 

the Questa Caldera and Their Potential 

Influence on Bedrock Ground-Water Flow, 

Red River Valley, New Mexico

By Jonathan Saul Caine

Prepared in cooperation with the New Mexico Environment Department

Professional Paper 1729

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Caine, Jonathan Saul, 2006, Questa baseline and premining ground-water quality investigation 18. Characterization of 

brittle structures in the Questa caldera and their potential influence on bedrock ground-water flow, Red River Valley, 

New Mexico: U.S. Geological Survey Professional Paper 1729, 37 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Physical Setting..............................................................................................................................................2

Geographic and Physiographic Features .........................................................................................2

Climate and Vegetation........................................................................................................................2

General Geology.............................................................................................................................................4

Mining History ................................................................................................................................................6

General Hydrology.........................................................................................................................................6

Surface Water.......................................................................................................................................6

Ground Water and Aqueous Geochemistry .....................................................................................6

Methods and Previous Work .......................................................................................................................7

Brittle Structures of the Questa Caldera ...................................................................................................8

Joint Networks......................................................................................................................................8

Faults, Fault Zones, and Fault-Related Fracture Networks ....................................................................8

Low-Angle Structures........................................................................................................................13

High-Angle Composite Deformation Zones ...................................................................................15

High-Angle Distributed Deformation Zones ..................................................................................18

Other Potential Structural and Hydrogeologic Heterogeneities in the Questa Caldera......................22

The Caldera Margin and Ground-Water Flow................................................................................22

Stockwork and Other Vein Systems ................................................................................................22

Lithologic Contacts.............................................................................................................................22

The Range-Front Fault Zone..............................................................................................................23

Faults in Surficial Deposits................................................................................................................23

Insights from Estimation of Bedrock, Caldera Margin, and High-Angle Fault-Zone 

Permeability Structure ..................................................................................................................23

Introduction to Computations ...........................................................................................................23

Computational Results for the Bedrock ..........................................................................................26

Computation and Results for the Caldera Margin and High-Angle Small Faults .....................28

Summary and Discussion of Potential Structural Controls on Bedrock Ground-Water Flow ........31

Acknowledgments.......................................................................................................................................35

References Cited..........................................................................................................................................35

Figures

1. Location and topographic map of the study area, showing alteration scars (yellow), 

debris fans (blue), known ore bodies (red), the mine site (pink), and the open pit 

(black dashed line) ......................................................................................................................3

2. Simplified geologic map of the southern portion of the Questa caldera in the vicinity 

of Red River, New Mexico...........................................................................................................4

3. Southwest section of the simplified geologic map of the Questa caldera shown in 

figure 2............................................................................................................................................9
iv

4–7. Photographs:

4. View looking south at open joints with simple orthogonal orientations at the Tsed 

locality a few meters above the main Red River channel....................................................10

5. Oblique view of complex orthogonal open joints at the Amalia Narrows locality at 

the edge of the main stream channel......................................................................................10

6. View looking down on joints with conjugate geometry at the Small Scar locality .........11

7. Example of a high-intensity, highly interconnected joint network found in the Pit Ore 

Body locality ................................................................................................................................11

8. Conceptual diagram of distinctive fault-zone architectural styles found in nature........13

9. Geologic map draped onto a 30-meter digital elevation model with no vertical exaggeration and a 100-foot contour interval ................................................................................14

10–20. Photographs:

10. Low-angle fault zone looking approximately west, showing Tertiary rhyolite and 

boulder conglomerate in the upper plate and Precambrian granite in the lower 

plate. .............................................................................................................................................15

11. Fault damage zone of the low-angle fault zone.....................................................................16

12. Closeup view of phacoidally cleaved, white, red, and black, clay-rich fault gouge in 

the low-angle fault-zone core ..................................................................................................16

13. Closeup view of silicified, polished, and striated slip surfaces that mark the contact 

of the upper and lower plates of the low-angle fault zone..................................................17

14. View looking south of the high-angle fault zone that intersects the low-angle fault 

zone at the lower left-hand corner of photograph................................................................17

15. View looking north at the fault core of the high-angle fault zone ......................................19

16. Manganocrete deposit (iron and manganese oxide-cemented alluvial and colluvial 

materials) at the base of the Cabin Springs cliffs on the north bank of the Red River....19

17. View looking north at the Cabin Springs cliff rhyolite dike that strikes parallel to the 

plane of the photograph and is cut by open and relatively small extensional faults ......20

18. View looking north at small faults, fault-related fracture networks, and nearorthogonal joint sets in the Bear Canyon pluton at the mountain front and the west 

end of the Red River Valley .......................................................................................................20

19. View looking north at the fault core of a small fault in the Cabin Springs cliffs ..............21

20. Small extensional fault and primary slip surface in the lower pit ore body showing 

“moly paint” and iron oxide staining that is probably related to recent seepage, and 

open fractures in the footwall damage zone .........................................................................21

21. Segment of a geologic cross section from the geologic map of the Latir volcanic 

field including the Questa caldera...........................................................................................22

22. Diagram showing two-dimensional, cross-sectional ground-water flow net for a 

region with high gradients and an isotropic, homogeneous bedrock aquifer at the 

bottom of which there is a no-flow boundary........................................................................23

23. Schematic diagram showing an average topographic profile from the north divide of 

the Red River watershed to the river, average potential topographic head gradients 

for varying depths to water at the divide, and the geometry and parameters used ......26

24. Pseudo three-dimensional geologic and topographic map draped on a 30-meter 

digital elevation model with no vertical exaggeration showing the model setup for 

computations of hydraulic conductivity for the bedrock aquifer system on the north 

divide.............................................................................................................................................28

25. Plot of computed hydraulic conductivity (K) values relative to depth of crosssectional areas for estimated gradients and values of estimated bedrock discharge to 

the Red River alluvial aquifer. ...................................................................................................29
v

26. Pseudo three-dimensional geologic and topographic map draped on a 30-meter 

digital elevation model with no vertical exaggeration .........................................................30

27. Plot of computed K values relative to depth of fault damage zones for gradients and 

values of estimated discharge to the Red River alluvial aquifer system...........................31

28. Combined plot (plots 25 and 27) of computed K values relative to geological feature 

cross-sectional area depth for the bedrock alone and for fault damage or distributed 

deformation zones. .....................................................................................................................32

29. Fifteen-meter resolution, natural-color satellite image draped on a USGS digital 

elevation model...........................................................................................................................33

Tables

1. Joint intensity data .......................................................................................................................9

2. Fault-related damage zone fracture intensity data...............................................................12

3. Small fault intensity data ...........................................................................................................18

4. Reported bedrock hydraulic data for the region around the Questa mine and the 

USGS Straight Creek research site .........................................................................................24

5. Input data and summary statistics for computed and reported bedrock and fault 

zone hydraulic properties..........................................................................................................27
Abstract

This report presents a field-based characterization of 

fractured and faulted crystalline bedrock in the southern portion of the Questa caldera and its margin. The focus is (1) the 

identification and description of brittle geological structures 

and (2) speculation on the potential effects and controls that 

these structures might have on the potential fluxes of paleo 

to present-day ground water in relation to natural or miningrelated metal and acid loads to surface and ground water. The 

entire study area is pervasively jointed with a few distinctive 

patterns such as orthogonal, oblique orthogonal, and conjugate 

joint sets. Joint intensity, the number of joints measured per 

unit line length, is high to extreme. Three types of fault zones 

are present that include partially silicified, low- and high-angle 

faults with well-developed damage zones and clay-rich cores 

and high-angle, unsilicified open faults. Conceptually, the 

joint networks can be thought of as providing the background 

porosity and permeability structure of the bedrock aquifer 

system. This background is cut by discrete entities such as 

the faults with clay-rich cores and open faults that may act as 

important hydrologic heterogeneities. The southern caldera 

margin runs parallel to the course of the Red River Valley, 

whose incision has left an extreme topographic gradient at 

high angles to the river. Many of the faults and fault intersections run parallel to this assumed hydraulic gradient; thus, 

these structures have great potential to provide paleo and 

present-day, discrete and anisotropic pathways for solute 

transport within the otherwise relatively low porosity and 

permeability bedrock background aquifer system. Although 

brittle fracture networks and faults are pervasive and complex, 

simple Darcy calculations are used to estimate the hydraulic 

conductivity and potential ground-water discharges of the bedrock aquifer, caldera margin, and other faults in order to gain 

insight into the potential contributions of these features to the 

ground-water and surface-water flow systems. These calculations show that, because all of these features are found along 

the Red River in the Cabin Springs−Columbine Park−Goat 

Hill fan area, their combined effect increases the probability 

that the bedrock aquifer ground-water flow system provides 

discharge to the Red River along this reach.

Introduction

The Questa caldera is exceptionally well exposed in 

the Red River Valley of northern New Mexico and is host to 

important metallic mineral deposits, particularly molybdenite 

(fig. 1). These deposits, mined since the late nineteenth century, are associated with extensive and noneconomic hydrothermal alteration and pyritization of the country rock. Both 

natural and mining-related trace-metal and acid loading to 

surface and ground water occurs in the Red River Valley. The 

overall purpose of this multireport, baseline investigation is 

to characterize the geological, hydrological, and geochemical 

attributes of the Red River Valley that control the premining 

ground-water quality. The purpose of this particular report is 

the characterization of the crystalline bedrock aquifer in the 

Red River Valley where the occurrence, storage, and flow of 

ground water, the solutes within it, and its interactions with 

surface water and the Molycorp molybdenite mine are largely 

controlled by fracture networks and fault zones. This report 

also complements other U.S. Geological Survey (USGS) 

reports on the geological, geochemical, geophysical, and 

hydrological characteristics of the lower Red River watershed 

(Lucius and others, 2001; Livo and Clark, 2002; Briggs and 

others, 2003; Caine, 2003; McCleskey and others, 2003; Powers and Burton, 2003; LoVetre and others, 2004; Ludington 

and others, 2004; Naus and others, 2005; and Nordstrom and 

others, 2005).

Field-based study of fracture networks and fault zones in 

crystalline bedrock helps to characterize the potential physical heterogeneities that may control the bedrock ground-water 

Questa Baseline and Premining Ground-Water Quality 

Investigation 18. Characterization of Brittle Structures 

in the Questa Caldera and Their Potential Influence 

on Bedrock Ground-Water Flow, Red River Valley, 

New Mexico

By Jonathan Saul Caine
 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

flow system. The southern margin of the Questa caldera 

provides a unique opportunity to complete such a study in a 

deeply eroded, mineralized, and hydrothermally altered caldera margin setting (figs. 1 and 2).

The Questa caldera is a complex hydrogeologic system containing four major, generic, hydrogeologic units of 

concern. These include, in order of inferred decreasing bulk 

porosity and permeability: (1) waste-rock piles associated with 

mining operations, (2) surficial materials such as Quaternary 

and Holocene sediments, (3) weathered bedrock, and (4) relatively unweathered bedrock. All of the bedrock in the study 

area is either crystalline intrusive, extrusive, or metamorphic 

rock (Lipman, 1983; Lipman and Reed, 1989). The primary 

unweathered or hydrothermally altered intergranular porosity and permeability of the rocks are low compared to many 

sedimentary rocks (for example, see Freeze and Cherry, 1979). 

The porosity and permeability of the Questa bedrock are 

fundamentally controlled by secondary brittle structures such 

as joints, faults, and fault zones. Mineralization in the form 

of veins and hydrothermal alteration zones and the depth of 

weathering of these brittle structures and hydrothermal alteration zones also affect the porosity and permeability structure 

of the bedrock, probably to diminishing degrees with depth 

(for example, Ludington and others, 2004). Understanding the 

occurrence, storage, and flow of ground water in the bedrock 

as controlled by secondary porosity and permeability, as well 

as their heterogeneity, is predicated on understanding the types 

and geometric extents of brittle structures in the study area. 

Rock units exposed in the study area represent a deeply eroded 

portion of the Questa caldera system (see Lipman, 1983). 

Therefore, the extent to which the structures described here, 

and their potential hydrogeological effects, are representative 

of calderas in general is unknown.

Physical Setting

Geographic and Physiographic Features

The study area is located in Taos County in the Taos 

Range of the Sangre de Cristo Mountains of north-central New 

Mexico (fig. 1). The Red River drainage basin is a tributary to 

the Rio Grande within the Carson National Forest. The area is 

a rugged, mineralized, and hydrothermally altered terrain with 

steep slopes and V-shaped valleys (fig. 1). The study reach of 

the Red River is in the Red River Valley between the gaging 

station at 2,280-m elevation at the west end and the town of 

Red River at 2,646-m elevation at the east end (fig. 1). The canyon 

walls of the Red River Valley rise steeply from 2,400 m to over 

3,000 m at the ridge crest on both the north and south sides of 

the river. Some peaks reach nearly 3,500 m. The 600-m difference in elevation between the Red River and the adjacent ridge 

crest can occur over a horizontal distance of less than 2,000 m. 

The Molycorp, Inc., Questa Molybdenite mine, referred 

to as the mine site, is located on the north side of New Mexico 

State Highway 38 and the Red River 13 km east of the mountain front and the mouth of the Red River canyon. The mine 

site is approximately 16 km2 and encompasses three main 

tributary valleys to the Red River: Capulin Canyon, Goat Hill 

Gulch, and Sulphur Gulch, from west to east, respectively (fig. 1). 

Mining activities produced extensive underground workings and an open pit of approximately 0.65 km2

 near or in Sulphur Gulch. Waste-rock piles cover steep slopes on the north 

side of the Red River between Capulin Canyon and Spring 

Gulch (a tributary valley of Sulphur Gulch). Hydrothermally 

altered bedrock is present in Capulin, Goat Hill, Sulphur, Hansen, Straight, June Bug, Hottentot, and Bitter Creek drainages 

(fig. 1). The latter five drainages are examples of unmined 

drainages with the exception of minor prospects in Bitter 

Creek. Weathering of extensively altered rock has resulted in 

steep, highly erosive, sparsely vegetated “alteration scars” that 

are clearly visible from the ground and in aerial photographs 

(Meyer and Leonardson, 1990). Where there has been no 

mining, these scar areas also are important natural sources of 

trace-metal and acid loading to surface and ground waters.

Climate and Vegetation

The Red River Valley lies within a semiarid desert that 

receives precipitation throughout the year and sustains moderate biodiversity. Between 1915 and 2002, the annual average 

temperature was 4°C, the precipitation and snowfall were 

52 cm and 370 cm, respectively, and the daily temperatures 

generally fluctuated by 18°C throughout the year (Western 

Regional Climate Center, 2003). 

Climate and vegetation vary greatly within short distances because of differences in topography, weather, bedrock, 

and sediment composition. Orographic effects of mountainous 

topography lead to precipitation on the windward slopes and 

localized storms within tributary valleys. Major precipitation events include summer thunderstorms and winter/spring 

snowstorms. Thunderstorms are responsible for mass wasting 

in hydrothermally altered scar areas, producing debris flows 

that may affect vegetation, alluvial aquifers, and the Red 

River. Winter snowpack contributes to ground-water recharge 

through snowmelt infiltration and runoff.

Prevalent vegetation in the Red River Valley is representative of the following altitude zones: piñon-juniper woodland 

(1,800−2,300 m), mixed conifer woodland (2,300−2,740 m), 

and spruce-fir woodland (2,740−3,660 m; Knight, 1990). 

Willows, cottonwoods, shrubs, perennial grasses, and flowering vegetation are common near the banks of the Red River. 

Extending from the river are widely spaced piñon pines and 

junipers. Gains in altitude first give rise to an abundance of 

ponderosa and limber pines and to Douglas and white fir at 

higher altitudes. This typical mountain community, although 

diverse, is dominated by ponderosa pines (Larry Gough, U.S. 

Geological Survey, oral commun., 2003).
Figure 1. Location and topographic map of the study area, showing alteration scars (yellow), debris fans (blue), known ore bodies (red

(pink), and the open pit (black dashed line). Topography from U.S. Geological Survey, 1:100,000 Wheeler Peak 30 × 60 quadrangle. Scar gGeoff Plumlee.
 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

General Geology

The geochemical interaction of water with soil, colluvium, alluvium, and fractured bedrock produces the composition of surface and ground waters that are the overall focus 

of this baseline and premining ground-water-quality investigation. Hence, the geology is a fundamental component to 

understanding the hydrology and ground-water chemistry. This 

section briefly summarizes the general geologic framework 

from Schilling (1956), Rehrig (1969), Lipman (1981), and 

Meyer and Leonardson (1990, 1997), in addition to observations made by the USGS scientists currently working at the 

site (Ludington and others, 2004). 

The Taos Range is composed of Precambrian metamorphic rocks and granitic intrusive rocks overlain and intruded 

by Tertiary volcanic and plutonic rocks (fig. 2). The volcanics 

are primarily of intermediate to felsic composition (andesites 

to rhyolites) and are intruded by late Oligocene and early 

Miocene quartz monzonites and granites that provided the 

source of the hydrothermal fluids and molybdenite mineralization. The hydrothermally altered volcanics commonly contain 

pyrite mineralization (generally 1−3 percent). The Red River 

Valley lies along the southern edge of the Questa volcanic 

caldera and contains complex structural features and extensive 

hydrothermal alteration associated with the evolution of the 

Rio Grande rift (figs. 1 and 2). The mineral deposits in the 

Red River Valley are considered Climax-type deposits that are 

associated with silica- and fluorine-rich rhyolite porphyry and 

granitic intrusives. The three principal alteration zones include 

highly altered quartz-sericite-pyrite (QSP), less-altered argillic 

(dominantly kaolinite) zones, and mildly altered propylitic 

zones (containing calcite mineralization). Quartz-sericitepyrite alteration, as the name implies, produces a mixture 

of quartz, pyrite (up to 10 percent), and fine-grained mica 

Figure 2. Simplified geologic map of the southern portion of the Questa caldera in the vicinity of Red River, New Mexico, modified 

from Lipman and Reed, 1989. The individual rock types shown are a starting point for conceptualizing what might be hydrologically 

significant units or domains in the study area and were used as a guide for collecting brittle-structures data. Note that the areas 

affected by hydrothermal alteration are shaded in transparent gray.
(sericite) or illite. Chlorite, epidote, albite, and calcite typically are present in the propylitic assemblages. Ore deposits 

contain quartz, molybdenite, pyrite, fluorite, calcite, manganiferous calcite, dolomite, and rhodochrosite. Lesser amounts of 

galena, sphalerite, chalcopyrite, magnetite, and hematite also 

are present. The hydrothermal alteration related to mineralization overprints an older, regional propylitic alteration. In these 

altered areas, rocks can contain a mixture of quartz, pyrite, 

and illite clays replacing feldspars, chlorite, carbonates, and 

epidote. Abundant minerals in waste rock produced by mining activities include chlorite, gypsum, illite, illite-smectite, 

jarosite, kaolinite, and muscovite (Gale and Thompson, 2001).

Andesite volcanic and volcaniclastic rocks are present in 

most scar-area bedrock outcrops and are the dominant bedrock 

units in the Straight Creek, South and Southeast Straight 

Creek, South Goat Hill, Sulphur Gulch, and Southwest Hansen 

scars (Ludington and others, 2004, and fig. 1). Amalia Tuff, 

a mildly alkaline, rhyolitic tuff, is the dominant rock type in 

the Goat Hill and Hansen scars, and quartz latite porphyry is 

the main rock type in the June Bug and Southeast Hottentot 

scars (fig. 1). Rhyolite porphyry is the main rock type in the 

Hottentot scar, and quartz latite and rhyolite porphyries form 

the hillslopes of many scars. Rhyolite porphyry and tuff do not 

seem to have been substantially affected by propylitization. 

Advanced argillic alteration was identified in the Hansen and 

Hottentot scars and in areas southwest of the Molycorp open 

pit. Propylitized andesite bedrock is present in several drainages that typically do not contain alteration scars.

After eruption of the Amalia Tuff and the formation of 

the Rio Grande rift 26 million years ago, the landscape of 

the study area consisted of a broad, low-relief plateau (P.W. 

Lipman, written commun., 2005). Rifting has caused as much 

as 7 km of structural relief across the Sangre de Cristo fault 

system that forms the eastern structural boundary of the rift 

and the western boundary of the Sangre de Cristo mountain 

block (Kluth and Schaftenaar, 1994; Grauch and Keller, 2004). 

Tectonically driven uplift caused the Red River to incise into 

the Taos Range resulting in the development of today’s rugged 

terrain (Kirk Vincent, U.S. Geological Survey, written commun., 2005). However, prior to a million years ago, the Red 

River was essentially the headwater of the Rio Grande and it 

flowed near the level of the top of the sediments within the 

rift basin (Wells and others, 1987). About 700 to 600 thousand 

years ago the San Luis Basin was captured by the Rio Grande, 

notably increasing discharges and incision in the Rio Grande 

and tributary watersheds and relegating the Red River to the 

status of one of those tributaries (Wells and others, 1987). 

Although thin Pleistocene alpine glaciers formed in the highest 

peaks of the Sangre de Cristo Mountains, they probably had 

little influence on the form of the Red River Valley. 

During the late Holocene, forested hillslopes eroded at 

about 0.04 mm/yr as compared to the then-exposed alteration 

scars, which eroded at about 3.0 mm/yr (Kirk Vincent, written 

commun., 2005). This extremely rapid erosion rate caused 

tributary watersheds containing alteration scars to deliver 

more sediment to the Red River than it could transport away, 

thus developing large debris fans to interfinger with alluvial 

sediments within the valley (Kirk Vincent, written commun., 

2005). These hydrothermally and structurally controlled facies 

relationships have resulted in significant heterogeneities within 

the generally high permeability Red River Valley alluvial aquifer because the fan deposits have several orders of magnitude 

lower permeability than the alluvial deposits.

Tectonic deformation combined with igneous and exhumation processes from Precambrian through Holocene times 

have produced a highly complex array of folds, fault zones, 

fractures, and veins. Since the inception of the caldera in the 

Oligocene, the region around the Red River Valley has been 

in a tensional stress regime that has ultimately produced the 

ancient and modern Rio Grande rift as previously discussed 

(Meyer and Foland, 1991; figs. 1 and 2). Generally northnorthwest- to north-northeast-striking extensional fault zones 

have been progressively reactivated during the coupled development of the caldera, the ore mineral system, and the eastern 

rift flank. These fault zones have accommodated approximately 200 percent extensional strain focused in the caldera 

(Meyer and Foland, 1991; fig. 2) and show a wide variety of 

internal deformation and structures, degree of mineralization, 

and dip. Numerous fracture networks, joints, and veins pervasively penetrate the entire study area. Collectively, these brittle 

structures form the permeability structure, in otherwise generally low permeability crystalline rock, in which the modern 

geochemical and ground-water flow system exists. 

Bedrock at the surface and buried under alluvial, colluvial, and anthropogenic deposits shows a variable and complex 

zone of weathering. Samples collected from a weathering profile in the Straight Creek scar were studied in detail to characterize the mineralogic variations in a weathered profile (see 

Ludington and others, 2004, and fig. 1). Relatively unweathered bedrock exposed in the creek bottom is propylitized 

andesite with a QSP overprint. Depending on location within 

the weathering profile, altered rocks contain variable amounts 

of quartz, illite, chlorite, and plagioclase feldspar, with smaller 

amounts of pyrite, gypsum, rutile, jarosite, and goethite 

(Livo and Clark, 2002; Ludington and others, 2004). Calcite, 

goethite, and sericite are widely distributed in the Red River 

Valley rocks and soils, as revealed by Airborne Visible/InfraRed Imaging Spectrometer data (AVIRIS) (Livo and Clark, 

2002). Calcite is an important mineral in the Red River Valley 

because its dissolution effectively neutralizes the acid inflows 

so that pH values in the Red River tend to be alkaline (pH 

7−8). Gypsum is common throughout the Red River Valley 

and forms as a secondary product of acid-sulfate weathering 

when pyrite oxidizes and reacts with calcite. Because gypsum 

is soluble in water, calcium and sulfate are the major ions in 

most surface and ground waters in the Red River Valley.

The major minerals in rock samples collected during mineral 

exploration and mining are biotite, calcite, chalcopyrite, fluorite, 

galena, molybdenite, pyrite, quartz, rhodochrosite, and sphalerite. Mining activities produced roughly 328 million tons of rock 

overburden in Capulin Canyon, along the north slope of the Red 

River, and in Goat Hill and Sulphur and Spring Gulches (URS, 

General Geology 
 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

written commun., 2001). The abundant minerals in waste-rock 

samples include chlorite, gypsum, illite, illite-smectite, jarosite, 

kaolinite, and muscovite (Gale and Thompson, 2001). 

Mining History

Prior to 1916, prospectors discovered outcrops of rich, 

yellow, weathered rock that looked like sulfur (hence the 

name, Sulphur Gulch) and a greasy mineral that looked like 

graphite (fig. 1). The graphitic mineral was identified as 

molybdenite in 1916, and the yellow mineral was ferrimolybdite, the oxidized product derived from the weathering 

of molybdenite (Schilling, 1956). By 1920 the Molybdenum 

Corporation of America had acquired the property to mine 

the deposit, and the company subsequently shortened its 

name to Molycorp, Inc. A mill and flotation plant was set up 

by 1923, and small-scale underground mining of high-grade 

veins (averaging 4 percent with a maximum of 35 percent) 

continued until 1958. During the 1950s exploration identified 

a large, low-grade deposit (about 0.3 percent) below the highgrade deposit, and the decision was made to extract this ore by 

open-pit methods (Carpenter, 1968). Extraction of open-pit ore 

began in 1965, and tailings were transported by a 9-mile-long 

pipeline to a tailings facility in the Rio Grande Valley just west 

of Questa. Peak annual production of 11.5 million pounds 

occurred in 1976. In 1977 Molycorp, Inc., became a whollyowned subsidiary of Union Oil Company of California.

Open-pit mining ceased in 1983 and underground mining 

restarted. Waste rock was no longer dumped onto piles at the 

mine site, but the volume of tailings transported in the pipeline to the Questa facility increased. Low market values for 

molybdenum have periodically caused the mine to shut down 

(1986−89 and 1992−95). Active mining continues at Molycorp’s Questa mine in response to market demand. Further 

history and related information on the Questa Molycorp mine 

are available from Molycorp, Inc. (www.molycorp.com) and 

from the U.S. Environmental Protection Agency (www.epa.

gov/superfund/sites/npl/nar1599.htm).

General Hydrology

Surface Water

The Red River originates at an altitude of approximately 

3,658 m near Wheeler Peak, the highest peak in New Mexico 

(4,011 m), and flows roughly 55 km to its confluence with the 

Rio Grande at an altitude of 2,012 m. Total basin drainage area 

is 492 km2, and the drainage area upstream from the Questa 

Ranger Station gaging station (at the mouth of the Red River 

Canyon) is 293 km2 (fig. 1). Peak streamflow usually occurs 

from late May to mid-June, with snowmelt-related flows beginning in late March and increasing through mid-April. Summer 

thunderstorms are common in July and August. Between 1930 

and 2001, the mean annual discharge of the Red River at the 

Questa Ranger Station gage has ranged from 0.36 to 2.92 m3/s, 

and the average daily discharge ranged from 0.07 to 21.24 m3/s 

with an average of 1.33 m3/s (U.S. Geological Survey, 2004).

The main drainages in the vicinity of the mine site are 

Capulin Canyon, Goat Hill Gulch, and Sulphur Gulch on the 

north side of the Red River (fig. 1). Upstream from the mine 

site, Hansen, Southwest Hansen, Straight, Hottentot, and Bitter Creeks drain scar areas, whereas Mallette Creek drains a 

nonscar area on the north side of the Red River. Bear Canyon, 

Columbine, Pioneer, and Placer Creeks drain largely unmineralized land on the south side of the river (fig. 1). Bear Canyon 

and upper Pioneer Creeks also contain some mineralization. 

Downstream from the mine site and in the Rio Grande Rift 

Valley, the Red River joins with Cabresto Creek, which enters 

from the north, before it discharges to the Rio Grande.

Springs and shallow alluvial ground water discharge to 

the Red River, rendering it a gaining stream over much of 

its length (Smolka and Tague, 1989). Between the town of 

Red River and the gaging station near Questa, there are about 

25 ephemeral seeps and springs along the banks of the Red 

River and approximately 20 intermittent seeps and springs in 

tributary drainages on the north side of the river (South Pass 

Resources, Inc., written commun., 1995; Steffen, Robertson, 

& Kirsten, written commun., 1995; Robertson GeoConsultants, Inc., written commun., 2001). Most seeps and springs 

are acidic (pH 2−4) with high conductance, dissolved solids, 

and metal concentrations. Aluminum hydroxide often precipitates from springs downgradient from scar and mined areas on 

the north side of the Red River, affecting the color and turbidity of the river (Vail Engineering, Inc., 1989).

Ground Water and Aqueous Geochemistry

Aquifer and other geologic and anthropogenic units in 

the Red River Valley are highly varied. Waste-rock piles and 

scars with associated debris fans are geochemically reactive, 

have high porosity, and have relatively high permeability. 

Some of the permeability may form from transient desiccation processes, episodic mass movements, or creep that form 

irregular and fracturelike openings at a wide range of scales in 

these poorly consolidated surficial materials. Alluvial aquifers 

are restricted in areal extent and have variable compositions. 

Hillslope soils are thin and composed of materials eroded 

from adjacent upgradient slopes. Debris fans are composed 

of sediments rapidly shed from their respective watersheds. 

Where the tributary watersheds contain “alteration scars,” the 

debris fans are large and active and contain both coarse and 

fine-grained, largely unsorted, clay-rich, debris-flow sediments, 

making them heterogeneities in the alluvium, as previously 

discussed. 

The chemistry of these sediments reflects the chemistry 

of their source terrains. The rapidly eroding and altered erosion scars have source materials that can generate low-pH, 
trace-metal-rich ground water. Sediments deposited by the Red 

River, in contrast, generally consist of well-washed, rounded 

sands, gravels, and cobbles and are composed of a mix of 

lithologies present in the entire Red River basin, including 

carbonates from the uppermost reaches. The largest debris fans 

caused the Red River alluvium to aggrade behind the fans during the Quaternary. Thus, ground water flowing in the shallow 

alluvial aquifer passes alternately through Red River alluvium and debris fans and may emerge when a relatively low 

permeability fan is contacted. Both the Red River alluvium 

and debris fans as a complex aquifer unit are less than several 

hundred meters wide and less than 100 m thick.

Alluvial ground water is a calcium-sulfate water type 

with magnesium commonly the second most abundant cation. 

Ground water downgradient from the waste-rock dumps and 

scars has acidic pH values and elevated metal concentrations 

compared to ground water upgradient from these altered areas. 

The bedrock aquifer constitutes the largest mass of aquifer in the study area but probably contains only relatively small 

amounts of ground water below the weathered zone because of 

inferred low porosity and low permeability largely controlled by 

fracture networks. Bedrock ground waters are also calciumsulfate type but usually of neutral pH. Most wells developed 

in the Red River Valley were installed to monitor water quality 

downgradient from mining operations (waste-rock dumps and 

tailings piles) and(or) scar areas. Wells installed in the Straight 

Creek drainage (fig. 1) during this study were developed for the 

purpose of measuring water levels and collecting water-chemistry samples for a range of environments in the Red River Valley, 

similar to the mine environment, and to interpret the water/rock 

interactions under nonmining baseline conditions as a reference 

for premining water/rock interactions for the mine site.

Hydrothermal alteration produced substantial changes in 

mineralogy over relatively short distances, a common feature 

of hydrothermally altered terrains. Hence, both the mineralogy 

and the resultant water chemistry can change substantially on a 

small spatial scale. Such hydrogeochemical environments are 

described as highly heterogeneous, making the estimation of 

premining ground-water chemistry a highly variable function of 

changing lithology rather than single fixed concentrations for an 

area the size of the mine site.

Methods and Previous Work

Field data including outcrop scale measurements and 

observations and descriptions of geological structures and host 

lithology were collected from representative outcrops of each 

of the major bedrock lithologic groups in the study area (see 

Caine, 2003). These data characterize brittle structures and 

document the major potential structural heterogeneities that 

may impart potential anisotropy, or possible fast pathways 

and preferential flow paths, for the transport of contaminants 

from the bedrock to surficial deposits or water bodies such 

as the Red River. Specific data collected in this study include 

fault zone, caldera margin, lithologic contact, and fracture 

network geologic and geometric properties such as orientation, 

composition, mineralization and alteration, intensity, length, 

shear sense and relative age, width, and shape. Numerous rock 

samples were collected (see Caine, 2003, for the computer 

file of the raw data). Explanations of the methods used in 

this characterization are provided in Caine and others (1996), 

Caine (2001a, b, c), Caine (2003), and Caine and Tomusiak 

(2003). Site-specific hydraulic data, tracer studies, aquifer 

pumping tests, borehole geophysical logging, and thermal and 

geochemical measurements should be conducted to verify the 

field data and associated interpretations. 

Previous work by Carpenter (1968), Rehrig (1969), Lipman (1983), Lipman and Reed (1989), and Meyer and Foland 

(1991) has established the foundation of the exceptionally 

complex bedrock geology of the study area. Meyer and Leonardson (1997) have added to this foundation through highly 

detailed outcrop-scale mapping at 1:6,000, the identification of 

numerous brittle structures, and a description of the geologic 

history and processes they represent. Several unpublished 

consultant reports also have contributed to understanding the 

bedrock hydrogeologic system. This report is the first known 

outcrop-scale characterization of brittle structures in bedrock 

in the specific context of their potential effects on the paleo to 

present-day ground-water flow system of the Questa caldera.

For the purpose of characterization of brittle structures, 

the bedrock geology can be broken into several significant 

rock groups that may have unique brittle structural attributes 

and that also might make those rock groups significant hydrogeological and hydrogeochemical units (see fig. 2, ensuing 

descriptions, and Ludington and others [2004], for a detailed 

description of the geology of the study area). These units 

include (1) Precambrian quartzo-feldspathic metamorphic 

rocks, (2) Tertiary precaldera volcanic rocks that are dominantly andesitic flows, (3) Tertiary Amalia Tuff, (4) Tertiary 

quartz latite flows, (5) Tertiary megabreccias, (6) Tertiary 

rhyolite intrusives, (7) Tertiary granitic intrusive rocks, 

(8) Tertiary siliciclastic sediments, and (9) numerous types of 

surficial deposits that were not studied as part of the bedrock 

characterization (Ludington and others, 2004). Structural and 

lithological data were collected from representative outcrops 

of each of these rock groups except the quartz latite flows 

to provide a the foundation for understanding the potential 

effects on the ground-water flow system.

On the basis of reported mass-balance calculations, an 

unaccounted-for ground-water discharge was identified along 

the Cabin Springs reach of the Red River, near an intersection 

of the caldera margin with the Red River (Kirk Vincent, written 

commun., 2005). To evaluate whether or not this discharge could 

come from the bedrock alone, from the caldera margin, from one 

or more of the fault zones, or from some combination of these, a 

set of simple Darcy calculations was performed, and the results 

are presented in this report. Using measured potential hydraulic gradients and cross-sectional areas and the unaccounted-for 

discharge values, theoretical values of hydraulic conductivities 

were calculated and compared with values reported from aquifer 

Methods and Previous Work 
   Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

tests from bedrock in the Questa mine area. Theoretical hydraulic conductivity values for the caldera margin, faults, and fault 

zones were compared with numerical model results for modeled 

fault zones as reported in the literature. This set of calculations, 

although oversimplified, demonstrates that either the bedrock 

alone, the caldera margin, an individual and narrow open extensional fault, or some combination of these, could produce the 

unaccounted-for discharge. These results illustrate the important 

point that often no unique solutions exist when making links 

between geological inference and hydraulic reality, particularly 

in highly complex geological environments. However, these 

results provide a starting point in setting up testable hypotheses 

that illustrate what the most important physical conditions and 

heterogeneities might be, regarding the interactions of mass flux 

from bedrock to surficial deposit aquifers, to the open pit and 

underground mine workings. 

Brittle Structures of the Questa Caldera

Joint Networks

Joints are fractures whose primary direction of opening is 

perpendicular to their walls and that are not filled with mineral 

precipitates. Fractures filled with mineral precipitates will 

be referred to as veins. Joints are formed by tectonic, brittle 

deformation as well as uplift and exhumation. Although the 

observed joints are not filled with mineral precipitates, most of 

them are mineral stained by iron oxides, occasional manganese oxides, and rare molybdenite “paint.” Mineral staining typically is restricted to joint surfaces, except in areas of 

hydrothermal scars (Meyer and Foland, 1991; Ludington and 

others, 2004) and very likely attests to recent, fracturedominated ground-water flow in the bedrock.

Figure 3 shows contoured joint orientation data plotted on equal-area projections for each outcrop studied. The 

shaded regions are contoured poles to joints where the darker 

colors represent higher concentrations of joints in a particular 

range of orientations. Clusters of joints that share a common 

orientation with some dispersion about a mean orientation can 

be thought of as part of a “joint set.” Multiple joint sets form 

a joint network whose relative orientations, intensities, and 

apertures largely control the storage, magnitude, and direction 

of ground-water flow in the subsurface.

Joint networks range from simple to complex orthogonal 

sets as in the Tertiary sediments, Amalia Narrows, Amalia 

Road Cut, and Cabin Springs cliffs localities (for example, see 

figs. 4 and 5) to complex conjugate and random geometries 

such as those found in the Small Scar locality (fig. 6) and Pit 

Ore Body locality (fig. 7), respectively. In general, at each 

locality studied there is some combination of two high-angle 

sets, one striking northeast-southwest and one striking northwest-southeast and one subhorizontal set (fig. 3). This pattern 

is consistent with orientations measured from previous work at 

Questa (for example, Carpenter, 1968; Rehrig, 1969) as well 

as regionally (for example, Caine and Tomusiak, 2003).

All lithologies observed in the study area are pervasively 

jointed by high-intensity joint networks. Measures of joint 

intensity, or number of joints along a line or within an area, 

reported here are simple joint counts along scanlines that are 

perpendicular to joint-set average orientations. This type of 

measure tends to overestimate intensity but allows for simple 

counting with great rapidity and is thus appropriate for reconnaissance measures such as those applied in this study. Cumulative joint intensities of joint sets range from 55 joints per 

meter to 0.3 joint per meter with an average of 28 joints per 

meter (table 1 and figs. 4 and 7 as examples of the extremes in 

intensity).

Joint lengths are difficult to measure, but field estimates 

range from a few meters up to a few tens of meters. Numerous joints with trace lengths less than 1 meter exist in all rock 

types; however, these may be more significant for storage 

rather than flow of ground water. Joint shapes are generally 

curviplanar to planar, and their walls typically are smooth but 

may be as rough as the scale of the grains that make up the 

rock.

Joint apertures, or the joint wall perpendicular opening 

width, are probably the most elusive property to measure with 

any degree of meaning because joint widths vary greatly from 

what is observed in the surface and subsurface (for example, 

Snow, 1970). Yet aperture is also one of the most critical properties when linking joint-related geometric properties to their 

hydraulic properties (see Caine and Tomusiak, 2003, for a 

discussion). For the reconnaissance style of the work reported 

here, joint apertures were recorded merely as greater than or 

less than 1 millimeter. Nearly all observations indicated that 

open joints had apertures less than 1 millimeter, and it was 

exceptional to have apertures greater than a few centimeters.

Truncation refers to the way in which individual joints 

join or cross other joints. In general, most joints observed in 

all Questa lithologies were mutually crosscutting, with the 

exception that some of the subhorizontal sets truncated (or 

ended) against one of the subvertical sets, indicating that the 

subhorizontal sets are younger (Segall and Pollard, 1983).

Faults, Fault Zones, and Fault-Related 

Fracture Networks

Faults and fault zones in the Questa caldera display 

distinctive architectural components, or internal structures 

(fig. 8), that also are found in noncaldera settings. Fault-zone 

architectural components include a fault core, defined as the 

part of a fault zone where most of the strain is accommodated 

and that often is composed of fault rocks such as clay-rich 

gouge or mineralized breccias or other cataclastic rocks 

(Caine and others, 1996). The second common component is 

a damage zone that often envelops the fault core and is related 

to the growth of the fault zone. Damage zones are commonly 
Table 1. Joint intensity data. 

[T, Tertiary; na, not available; max, maximum; min, minimum]

Lithology

Average number

open joints

by set

per meter

Max number

open joints

by set

per meter

Min number

open joints

by set

per meter

All sets cumulative

average

open joints

per meter

T scar na na na 13.5

T granite, submacroscopic na 55 na na

T granite, macroscopic 1.0 1.8 0.6 1.8

T rhyolite, cliffs 2.7 3.8 0.3 10.9

T rhyolite, pit ore body 5.2 6.6 1.7 20.9

T sediment 8.0 11.0 5.0 16.0

T Amalia 29.0 48.3 4.5 116.0

T megabreccia 15.8 18.0 13.3 47.3

T andesite, valley 7.1 8.7 4.0 21.2

T andesite, ridge 6.9 8.3 5.3 20.8

PC granite 3.9 12.0 2.0 11.6

 All localities 8.9 55 0.3 28.0

Figure 3. Southwest section of the simplified geologic map of the Questa caldera shown in figure 2 (modified from Lipman and Reed, 

1989). Fracture network and fault zone data-collection localities are shown. Zones of hydrothermal alteration shown in figure 2 have 

been removed for clarity. Poles to fractures are shown and contoured on equal-area projections using the method of Kamb (1959). All 

contour intervals are 2 sigma, and N = the number of fractures measured. Mean great circles for fault plane orientations and their mean 

poles are shown in red. All plots were generated with the computer program “Stereonet” by Allmendinger (1995).

Faults, Fault Zones, and Fault-Related Fracture Networks 
10 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

Figure 4. View looking 

south at open joints 

with simple orthogonal 

orientations at the Tsed 

locality a few meters 

above the main Red River 

channel. Poorly developed 

subhorizontal joints and 

vertical near perpendicular 

joints are also present 

in the outcrop. Note 

the accentuation of 

apertures in the vertical 

joints, probably due 

to weathering at the 

surface. The outcrop is 

approximately 3 meters 

wide.

Figure 5. Oblique view 

of complex orthogonal 

open joints at the Amalia 

Narrows locality at the 

edge of the main stream 

channel. Note the white 

precipitate along the 

margins of joints that 

may have formed due to 

surface tension as the 

stream receded after a 

recent storm or due to 

seepage from the joints 

themselves.
Figure 6. View looking 

down on joints with 

conjugate geometry at 

the Small Scar locality. 

Although extensively 

hydrothermally altered, 

the joints are open. 

Conjugate geometry of the 

joints suggests they are 

tectonic in origin.

Figure 7. Example of 

a high-intensity, highly 

interconnected joint 

network found in the Pit 

Ore Body locality. Note 

the discrete iron oxide 

stains. Although this is 

a highly interconnected 

network, the stains may 

be indicative of the highly 

discrete nature of the few 

hydraulically conductive 

fractures likely to be found 

in the subsurface groundwater flow system.

Faults, Fault Zones, and Fault-Related Fracture Networks 11
12  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

composed of open and mineralized fractures, small faults, 

folds and other strain features (Caine and others, 1996; Knipe 

and others, 1998). The core and damage zone are further 

enveloped by the relatively undamaged host rock or protolith. 

Fault-zone components not only are distinctive structural 

heterogeneities but also can be distinctive hydrogeological 

heterogeneities that may impart significant anisotropy within 

a ground-water flow system (Sibson, 1994; Caine and Forster, 

1999). Characterization of fault-zone architectural styles can 

be used to infer potential hydraulic behavior of faults where 

real hydraulic data (for example, from wells) are not available. 

The following field-based descriptions of the prominent types 

of fault zones in the study area are used later in this report to 

make simple Darcy calculations as estimates of fault-zone 

hydraulic behavior.

Fault-zone architectural styles, or combinations of each 

of the components, result in distinctive types of fault zones. 

Composite deformation zones (CDZ) have both common faultzone components: a core and a damage zone surrounded by 

the protolith. Example fault-related permeability structures 

might include the composite deformation zone where the core 

is low-permeability gouge and higher permeability open fractures in the damage zone, making the fault zone a combined 

conduit-barrier to ground-water flow where flow is enhanced 

parallel to the strike of the fault but impeded across the fault. 

Conversely, a distributed deformation zone (DDZ) may act 

as a conduit relative to the protolith if many of the internal 

fractures are open.

Characterization of fracture networks from surface exposures may not completely represent what is in the subsurface 

due to the near-surface formation of exhumation-related 

fractures. However, tectonic fractures, faults, fault zones, and 

fault-related fractures are likely to be well represented in outcrop exposures as these structures originally formed at depth 

unlike exhumation-related fractures. The aperture of fault and 

nonfault-related fractures, a major control on permeability, 

may change with depth and is dependent on the orientations of 

the fractures with respect to the magnitude of overburden load 

as well as the magnitude and orientation of in-situ tectonic 

stresses at progressively greater depths (Davis and Turk, 1964; 

Barton and others, 1995). Depending on orientation relative 

to the stress field, some fractures may close down while others 

may become wider. Although no attempt has been made to 

account for stress-related changes in fracture aperture, the 

range of model results presented here using Darcy calculations 

likely covers the range of effects due to stress-related aperture 

changes from the surface to the depths encountered in the 

underground mine workings. 

The spacing or intensity of faults and fault zones, as 

well as their associated fracture networks, also is expected to 

be well represented in outcrop exposures and from geological mapping. Again, this is due to their tectonic origin that 

is largely unaffected by near-surface exhumation-related 

processes. Fault and fault-zone trace-length displacement 

relationships as well as theoretical fracture mechanics dictate 

that the approximate three-dimensional shapes of faults in the 

subsurface can be generally understood from characterization of trace length (Cowie and Scholz, 1992). Fault tip lines, 

the edge of a fault or fault zone where displacement goes to 

zero, are likely similar to rough-edged ellipses to rough-edged 

rectangles (Shipton and Cowie, 2001). Given the reasonable assumption that our fundamental understandings of the 

tectonic history and processes that form faults, fault zones, 

and fault-related fracture networks apply to these structures 

Table 2. Fault-related damage zone fracture intensity data.

[T, Tertiary; rhy, rhyolite; sed, sediment; na, not available; max, maximum; min, minimum; DZ, damage zone; HW, hanging wall; 

Avg., Average]

Fault zone, location, lithology

Avg. number 

open

fractures

by set

per meter

Max number

open

fractures

by set

per meter

Min number

open

fractures

by set

per meter

All sets

cumulative

average open

fractures

per meter

Low-angle fault, upper plate DZ, Tsed 15.3 16.7 14.0 30.7

Low-angle fault, upper plate DZ, Trhy 16.3 23.0 9.5 32.5

Low-angle fault intersection with high-angle 

 fault, upper plate and HW DZ, Tsed 10.5 13.0 8.0 21.0

Low-angle fault, lower plate DZ, PC granite 3.5 4.2 2.8 10.5

Small fault DZ, Tsed, Upper Cliffs 37.0 na na na

High-angle fault DZ, Trhy, pit ore body 70.0 na na na

High-angle-long trace length, small faults, Trhy, Cabin 

Springs cliffs 55.0 na na na
in the subsurface, most of the faults observed in the Questa 

caldera at the surface—high intensity in their presence with 

extensive 10s to 100s of meters trace lengths and meters to 

100s of meters of displacement—extend well into the bedrock 

and are likely also present at depth with geometric, structural, 

and hydrological characteristics similar to what is observed in 

outcrop.

On the basis of field observations it appears that the 

major fault zones in the Questa caldera are either composite 

deformation zones or relatively narrow, distributed deformation zones. Host-rock lithology at Questa clearly has a large 

influence on fault-zone architectural style, as does the type of 

fault. Two end-member types of fault zones are found in the 

study area, consistent with mapping and observations made 

by Lipman and Reed (1989), Meyer and Foland (1991), and 

Meyer and Leonardson (1997). These include low-angle and 

high-angle faults, some of which form the primary structural 

and lithologic margin of the southern Questa caldera (figs. 2, 

3, and 9).

Low-Angle Structures

One outcrop of an exceptionally well exposed, east-weststriking, low-angle structure was found (J.W. Meyer, Hanover 

College, Santa Maria, California, oral commun., 2001). Other 

low-angle features were found in several localities (figs. 3 

and 9); however, only a few were faults, and most were either 

subhorizontal joints, volcanic unit flow boundaries, possible 

hydrothermal fluid conduits, or colluvial deposit contacts with 

bedrock. This observation is significant because there probably 

are only several low-angle, major fault zones in the study area, 

and thus they may be only highly localized hydrogeological 

heterogeneities most commonly associated with the caldera 

margin.

The well-exposed low-angle fault zone also is mapped as 

the caldera structural margin (Lipman and Reed, 1989; Meyer 

and Leonardson, 1997). Although the geological significance 

and history of this structure are important, further assessment 

is not within the context and focus of this report. Interested 

readers are referred to Meyer and Leonardson (1997) for a 

discussion on this topic. This structure shows an upper-plate 

damage zone composed of highly fractured, near-vertical 

septa of Tertiary rhyolite within the Tertiary pebble-to-boulder 

conglomerate (see figs. 3, 9, and 10 and compare Trhy and 

Tsed fracture intensities from tables 1 and 2). Fracture intensity in the rhyolite is uniformly higher than in the sediment 

where high-intensity fracturing is more localized. Fracturing 

in the upper-plate damage zone shows a strong clustering of 

high-angle, northwest-southeast- and north-south-striking 

open fractures. In stark contrast, the lower-plate damage zone, 

composed of Precambrian granite, shows fracture intensities 

no greater than its relatively undeformed protolith (tables 1 

and 2). 

As the fault core is approached from the damage zone, 

the deformation and hydrothermal alteration appear to increase 

in both the rhyolites and sediments in the upper plate and similarly in the Precambrian granite in the lower plate (figs. 10, 11, 

and 12). The damage-zone upper and lower contact is in sharp 

contact with the fault core and is demarcated by polished and 

striated, silicified slip surfaces with kinematic indicators dominantly indicating upper-plate, west to east motion; however, 

several steps on slip surfaces also indicated upper-plate east 

to west motion (figs. 3 and 13). This may indicate reactivated, 

incremental strain events and(or) possibly accommodation of 

local strain from emplacement of the Tertiary rhyolite dikes in 

the upper plate (fig. 10).

The central fault core itself is composed of foliated pods 

of highly sheared and mixed clay-rich gouge and cobbleconglomerate fault gouge in which some Tsed quartz-rich 

cobbles are essentially survivor clasts that are highly fractured 

Hill Fracture

Fault Meshes

Shawangunk

Mountains

San Gabriel

Cataclasites

Stillwater

Fault Zone

Fault Zone

Architectural

Styles

Single

Fracture

Fault

Distributed

Deformation

Zone

Composite

Deformation

Zone

Localized

Deformation

Zone

Figure 8. Conceptual diagram of distinctive 

fault-zone architectural styles found in nature 

(from Caine and Forster, 1999).

Faults, Fault Zones, and Fault-Related Fracture Networks 13
Figure 9. Geologic map modified from Meyer and Leonardson (1997) and draped onto a 30-meter digital elevation model with no vertical exaggerat

contour interval. Geologic units, geologic structures, their possible geomorphic expressions, and the Red River in the vicinity of Cabin Springs cliffs, the Goat Hill fan are shown. The locations of some outcrops studied in detail and for which there are photographs in the text are shown. These incluwell exposed low-angle structure and its intersection with the high-angle fault that cuts it and drops the caldera margin (red hachured line) down to Springs cliffs small fault network, and manganocretes on the north bank of the river. Discharge (Q) data from the USGS 2001 base-flow tracer dilutioand others, 2003) are shown as blue dots along the river. Values in cubic feet per second at the blue dots are the calculated Q value for that point. Tthe dots are the difference between the upstream and downstream values.
(figs. 11 and 12). The phacoidal (lens-shaped in three dimensions) cleavage is highly foliated with an anastomosing fabric 

subparallel to the dominant orientation of the major fault 

zone. This cleavage conforms into the bounding slip surfaces, 

leaving little open space. The magnitude of displacement on 

this fault is undetermined due to a lack of piercing points. The 

various gouges in the fault also appear to have been chemically altered, and no field-scale manganese oxides were found 

in this structure. 

The extensive, high-intensity, open-fracture networks in 

the upper-plate damage zone relative to the host rocks and the 

clay-rich fault core bounded by silicified slip surfaces suggest 

that this fault probably acts as a combined conduit-barrier to 

ground-water flow (fig. 8). Because of the few good exposures 

of the caldera margin in the study area, it is hard to determine 

how similar or different the structural architecture of other parts of the caldera margin may be, 

or if this type of structure is representative of 

other caldera structural margins in general.

High-Angle Composite Deformation 

Zones 

Directly adjacent to the low-angle structure 

is a high-angle, north-south-striking extensional 

fault that cuts and displaces the low-angle structure (figs. 3, 9, 13, and 14). The footwall of the 

high-angle fault is Precambrian granite, and the 

hanging wall is the upper plate of the low-angle 

structure. Below the upper plate/lower plate 

contact of the low-angle structure the rocks are 

largely buried, making it difficult to determine 

their interrelationship. However, a minimum 

dip-slip displacement (see figs. 3, 9, and 14) of 

14 meters, top down to the east, was measured 

between exposed Precambrian piercing points.

The footwall and hanging-wall damage 

zones in the adjacent high-angle fault zone are 

composed of open fractures and some quartz 

veins; however, the fracture intensities were not 

exceptionally higher than the background jointing in the granites. The damage-zone fracture 

orientations are mechanically compatible with 

an Andersonian-style high-angle extensional 

fault (fig. 3 and Caine and Forster, 1999). The 

fault core is composed of matrix-supported fault 

breccia and quartz-rich gouge with granite clasts 

surrounded by highly polished, striated, silicified, and corrugated slip surfaces into which the 

breccia and gouge are highly conformed, leaving 

little visible open space (fig. 15). An inner core 

also shows stringers and veins, subparallel 

to the fault core, that are composed of highly 

weathered iron and manganese oxides. These 

fault-core materials have undergone brecciation, 

suggesting that the oxides were originally emplaced prior to 

faulting and possibly are synmineralization. 

Local vuggy zones along the eastern fault-core damage-zone contact have clusters of centimeter-scale, calcite 

crystalline masses thickly coated with iron and manganese 

oxides. Few oxide mineral veins or stringers associated with 

faults were found anywhere in the vicinity of the core of this 

composite deformation zone. The only other place manganese 

oxide was found was as sparse coatings in small faults in rhyolite, discussed herein.

The architectural style of this fault can be classified as a 

composite deformation zone, as in Caine and Forster (1999). 

This architectural style may indicate that the fault zone is a 

present-day combined conduit-barrier, with ground-water flow 

enhanced parallel to the fault but impeded across it. Although 

Figure 10. Low-angle fault zone (the core is at the person’s shoulder) looking 

approximately west, showing Tertiary rhyolite (red) and boulder conglomerate 

(gray-brown) in the upper plate and Precambrian granite in the lower plate. The 

high-angle extensional fault discussed in the text is exposed in the far left side of 

the photograph. See figure 9 for map location.

Faults, Fault Zones, and Fault-Related Fracture Networks 15
16  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

Figure 11. Fault damage 

zone of the low-angle fault 

zone at the location of 

the person’s shoulder in 

figure 10. The lower plate is 

composed of hydrothermally 

altered Precambrian granite, 

and the upper plate in this 

photograph is composed of 

hydrothermally altered Tertiary 

boulder conglomerate. Note 

the multicolored, clay-rich 

fault gouge that separates the 

upper and lower plates.

Figure 12. Closeup view of 

phacoidally cleaved, white, 

red, and black, clay-rich fault 

gouge in the low-angle faultzone core. Note that the twodimensional long axis of the 

phacoidally cleaved gouge is 

parallel to the major orientation 

of the curviplanar primary slip 

surfaces and that this centralcore zone has a very sharp 

upper and lower contact with 

the upper- and lower-plate fault 

rocks and damage zones.
Figure 13. Closeup view 

of silicified, polished, and 

striated slip surfaces that 

mark the contact of the upper 

and lower plates of the lowangle fault zone. This slip 

surface is part of a complex 

network of slip surfaces that 

are both subhorizontal and 

subvertical but indicate westeast motion.

Figure 14. View looking 

south of the high-angle fault 

zone that intersects the 

low-angle fault zone at the 

lower left-hand corner of this 

photograph. Note the highly 

polished and striated white 

slip surfaces that form the 

footwall contact of the fault 

core and Precambrian granite 

footwall damage zone. See 

figure 9 for map location.

Faults, Fault Zones, and Fault-Related Fracture Networks 17
18 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

the fault-core silicified breccias are cemented, the presence of 

localized vuggy zones and oxidized veins or stringers indicates 

that there may have been, and still may be, highly localized 

ground-water flow within and parallel to the fault core. The 

observation that the fault-core oxide mineral veins are brecciated and thus possibly are synmineralization in origin strongly 

suggests there has been a persistent and localized source of iron 

and manganese present at this location prior to and during the 

postglacial formation of the Red River Valley. This is consistent 

with the presence of manganocretes, or manganese-oxidecemented alluvial deposits, along the banks of the Red River, 

particularly at Cabin Springs directly below this fault (fig. 16).

High-Angle Distributed Deformation Zones 

In contrast to the high-angle faults in the Precambrian 

granites and the low-angle fault system, there are numerous high-angle, north-south- to northwest-southeast-striking, 

extensional, small displacement faults in major rhyolite dikes 

and the Tertiary Bear Canyon granite pluton (figs. 3, 9, 13, 17, 

and 18).

Many of these high-angle, small fault zones also cut 

the Precambrian granite, but there were few outcrops where 

the styles of these faults could be directly characterized in 

Precambrian host rocks. The predominant difference in the 

Tertiary rhyolite small faults compared to the high-angle 

composite deformation zones in Precambrian granites and 

the low-angle caldera margin structure, discussed previously, 

is that the faults in rhyolite are dominantly open, unmineralized structures. The rhyolite small faults range in style from 

curviplanar single-fracture faults with no damage zones to 

bounded zones of open fractures up to 1 meter wide (figs. 8 

and 19). The cores of these faults are bounded by moderately 

polished and striated slip surfaces, and where dilated, the 

core is composed of extensive, distributed brittle fracturing 

of host rock with conjugate geometry; there is no fault gouge 

or breccia (fig. 19). Several of the high-angle rhyolite faults 

exhibit sparse and spotty, black mineral surface coatings that 

are dendritic and indicative of the drainage of manganese 

oxide-rich water through these faults. This staining is not 

part of or within the fault rocks as they are in the high-angle 

composite deformation zone described previously.

Displacement along these faults, both synthetic and 

antithetic in comparison with the high-angle composite 

deformation zone discussed herein, is less than about 2 meters. 

Particularly extensive, dominantly high-angle, north-south- to 

northwest-southeast-striking, distributed deformation in the 

form of open fault-related fractures was observed in the tip 

zones of this style of fault. In one particularly well-exposed 

cliff face (the Cabin Springs cliffs, figs. 9 and 17), fault intensity was measurable along a linear traverse, indicating that 

there was approximately one high-angle, small displacement 

fault every 10 meters on average (table 3; note that in fig. 9 

not all of the small faults observable in the cliff were mapped 

by Meyer and Leonardson, 1997). The architectural styles of 

these small faults characterize them as distributed deformation 

zones (fig. 8) and suggest they act as important conduits for 

ground-water flow parallel to their strikes. 

An exception to the open nature of the fault cores in the 

rhyolites is the small faults in the ore body itself. Figure 20 

shows a well-developed small fault in the rhyolite ore body 

found in the Molycorp pit. The fault core is composed of a 

molybdenite-infilled cataclasite, and the slip surface is striated 

“moly paint” as commonly observed by Carpenter (1968), 

Clark (1968), and Rehrig (1969). However, several of these 

faults do show narrow, iron-oxide-stained damage zones with 

some open fault-related fractures as shown in figure 20, making them composite deformation zones whose architecture is 

likely to be related to their proximity to the main ore body.

Table 3. Small fault intensity data. 

[T, Tertiary; rhy, rhyolite; sed, sediment]

Location, fault type, lithology

Total number

open faults

observed

Traverse

length

(meters)

Average number

open faults

per 10 meters

Cabin Springs cliffs, high-angle, long trace length,

 small displacement faults, Trhy and Tsed 11 66.9 1.6

Upper cliffs, high-angle, long trace length, small

 displacement faults, Trhy and Tsed 9 43.8 2.1
Figure 15. View looking 

north at the fault core of the 

high-angle fault zone. Note the 

silicified breccia in the core.

Figure 16. Manganocrete deposit (iron and 

manganese oxide-cemented alluvial and colluvial 

materials) at the base of the Cabin Springs cliffs on 

the north bank of the Red River. See figure 9 for map 

location. Photograph courtesy of Phil Verplanck.

Faults, Fault Zones, and Fault-Related Fracture Networks 19
20 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

Figure 17. View looking 

north at the Cabin Springs 

cliff rhyolite dike that strikes 

parallel to the plane of the 

photograph and is cut by 

open and relatively small 

extensional faults that strike 

perpendicular to the plane 

of the photograph. Note the 

subhorizontal basalt dike, 

shown by the red arrow, 

that cuts the rhyolite and is 

cut by the high-angle faults 

showing offsets on the order 

of a meter. See figure 9 for 

map location.

Figure 18. View looking 

north at small faults, faultrelated fracture networks, 

and near-orthogonal joint 

sets in the Bear Canyon 

pluton at the mountain front 

and the west end of the Red 

River Valley.
Figure 19. View looking 

north at the fault core of 

a small fault in the Cabin 

Springs cliffs. Displacement 

is less than 1 meter, and 

the fault core is composed 

of open and extensive, 

conjugate-style, fractured 

rhyolite host rock.

Figure 20. Small 

extensional fault and 

primary slip surface in the 

lower pit ore body showing 

“moly paint” and iron oxide 

staining that is probably 

related to recent seepage 

and open fractures in the 

footwall damage zone.

Faults, Fault Zones, and Fault-Related Fracture Networks  21
22  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

Other Potential Structural and 

Hydrogeologic Heterogeneities in the 

Questa Caldera

The Caldera Margin and Ground-Water Flow

Although exposures of the structural margin of the Questa 

caldera are minimal in the central and western Red River Valley, observations from mapping and the exceptional low-angle 

fault-zone exposure discussed previously show that this margin 

is a major lithologic and structural heterogeneity (Lipman and 

Reed, 1989; Meyer and Leonardson, 1997; figs. 9 and 21). Due 

to a lack of exposure, of drilling records from boreholes, or of 

data from mine workings that may have penetrated this margin, 

its true architectural form and the consistency of that form are 

unknown. However, one might speculate that, because of the 

displacement accommodated along this margin, its coincidence 

with intrusive and extrusive igneous activity, and its major 

lithologic juxtaposition, the margin is a highly complex hydrogeological feature that probably varies spatially between any 

one of the major end-member permeability structures—barrier, 

conduit, or combined conduit-barrier (fig. 8). 

The Red River Valley east-to-west downstream topographic gradient is steep (average about 0.02) and the northsouth valley walls are steeper yet (north side average about 0.2, 

figs. 9 and 22). Because these topographic gradients translate 

into potential hydraulic head gradients that drive ground-water 

flow, ground-water flow lines should converge along the thalweg of the Red River (for example, Hubbert, 1940, and fig. 22) 

and in three dimensions should have components of downvalley, lateral flow as well as upward flow from the bedrock to the 

stream.

The caldera margin crosses the Red River in three places 

as mapped (figs. 2 and 9), and given the previous observations 

regarding the brittle deformation associated with the margin 

where it is exposed, one might surmise that it could act as a 

zone of enhanced bedrock ground-water flow. However, the 

actual extent of bedrock ground-water discharge to the Red 

River is dependent not only on the gradients but on the bulk 

permeability structure of the bedrock itself relative to the bulk 

permeability of features such as the margin structure, faults 

that might cut it, and the overlying surficial deposits within the 

thalweg of the river.

Stockwork and Other Vein Systems

Several other geologic structures were observed that may 

be significant with respect to the Questa ground-water flow 

system. These include stockwork-style quartz and pyrite veins 

in Tertiary andesite as well as planar to curviplanar, manganese oxide-stained, north-south-striking quartz vein swarms 

in the Precambrian granite. The stockwork veins were pervasive in localized sections of andesite found in the thalweg of 

Straight, Hottentot, and two unnamed creeks. The quartz vein 

swarms were found in only two out of about eight outcrops of 

Precambrian granite, indicating they are localized. Numerous 

opening-mode and fault-related molybdenite veins and faults 

were also observed in ore-body-related Tertiary rhyolite in the 

Molycorp pit, as noted herein. The closed veins showed the 

full gamut of orientations but showed a dominance of northeast-southwest and northwest-southeast strikes and high to 

moderate dip angles, as noted in detail by Rehrig (1969).

Lithologic Contacts

Although few lithologic unit contacts were observed, 

these features may be hydrogeological heterogeneities due to 

open discontinuities between them. In the caldera system there 

are likely to be complex unit contacts that may fall into two 

Figure 21. Segment of 

a geologic cross section 

from the geologic map 

of the Latir volcanic field 

including the Questa 

caldera from Lipman and 

Reed (1989). Although 

this section is from the 

northern part of the 

caldera, the margin is 

clearly noted as a major 

lithological heterogeneity 

and extensional fault, and 

it is likely to be similar to 

that found in the Red River 

drainage.

N S
end-member styles: intrusive and extrusive or flow contacts. 

Observed intrusive contacts showed no open space and thorough welding. Joint networks that cut these contacts generally 

were not perturbed by the contact itself, and thus this type 

of contact may be transparent hydrologically (see Caine and 

Tomusiak, 2003). In contrast, extrusive contacts such as those 

found between various types of pyroclastic, andesite, or debris 

flows deposited on previously cooled units may be sufficiently 

rough as to provide open space for ground-water flow and thus 

may be localized conduits.

The Range-Front Fault Zone

At the mountain front to the west of the Molycorp mining 

operation, the Red River crosses a poorly exposed range-bounding extensional fault zone (Lipman and Reed, 1989, and fig. 3). 

Although the poor exposure does not allow for direct observation of the core or cores of this fault zone, one can observe what 

probably is part of the footwall damage zone in the westernmost 

exposures of the Bear Canyon pluton (figs. 3 and 18). As previously discussed, numerous small faults as well as pervasive, 

high-intensity joint networks are present.

Faults in Surficial Deposits

Although beyond the scope of this study, faulting in the 

alluvial and colluvial materials and their potential interactions with bedrock fault zones may be an important part of 

the hydrogeological story in the Red River Valley, particularly 

at the range front. If presently active surface-rupturing faults 

along the range-front fault system cut recent sediments of the 

Santa Fe Group and(or) younger surficial deposits, these active 

faults also may be involved with controlling mountain block, 

mountain front, and in-stream recharge processes in the Rio 

Grande basin. Presently active faults in the basin-fill sediments 

may be important heterogeneities in the context of solute 

transport of acidic and metal-rich water to the basin aquifer 

system. No attempt, however, has been made in this study to 

characterize such fault zones.

Insights from Estimation of Bedrock, 

Caldera Margin, and High-Angle FaultZone Permeability Structure

Introduction to Computations

Common conceptual models for fractured, crystalline bedrock 

aquifer systems include the idea that there is a highly fractured and 

weathered upper, high-conductivity zone that grades into lower and 

lower conductivity rocks with depth (Davis and Turk, 1964; Snow, 

1972; URS, written commun., 2001; Caine and Tomusiak, 2003). 

Although this type of bedrock aquifer system commonly has low 

bulk hydraulic conductivity, it can be heterogeneous and have zones 

of considerable conductivity, as shown in table 4, from aquifer 

hydraulic tests in the area of the mine as well as being consistent 

with values cited for other fractured bedrock aquifer systems (for 

example, approximately 10-4 to 10-8 m/s as reported in Freeze and 

Cherry, 1979). Characterization of the permeability structure of 

a complex bedrock aquifer system, such as that found in the Red 

River watershed, should ideally be based on (1) hydraulic data 

from wells derived from pumping tests within those wells, (2) time 

series water-level data from wells, (3) carefully designed measurements of natural and perturbed ground-water flow from mine 

tunnels initiated at the time the mine was originally constructed to 

monitoring through the life of the mine, (4) ground-water flow and 

discharge data from stream base flow and spring measurements, 

(5) surface meteorological data such as precipitation and evapotranspiration data, (6) geochemical data that can help constrain 

unique water/rock interactions between geological units or features, 

(7) geological data that identify potentially significant geological 

features that might cause heterogeneity within the aquifer system 

at various scales or that tie hydraulic data to geological reality, and 

(8) numerical ground-water flow modeling at appropriate scales 

with appropriate levels of detail that are used to progressively 

integrate the various types of data cited herein and to investigate the 

effects of simple and complex model realizations in order to better 

understand the system as a whole. Unfortunately, at Questa, many 

of the cited data are not available, and thus the use of simple Darcy 

calculations is warranted as a means of obtaining a first-order 

Figure 22. Two-dimensional, cross-sectional 

ground-water flow net for a region with high 

gradients and an isotropic, homogeneous 

bedrock aquifer at the bottom of which 

there is a no-flow boundary (after Hubbert, 

1940). Hydraulic theory dictates that in three 

dimensions there will be flow lines coming out 

of the page due to an up-gradient hydraulic 

head associated with higher and lower regions 

along the valley thalweg. 

Other Potential Structural and Hydrogeologic Heterogeneities in the Questa Caldera System  23
24  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

Table 4. Reported bedrock hydraulic data for the region around the Questa mine and the USGS Straight Creek research 

site.

[* See References Cited for footnoted sources. ** Test types: S, slug; P, single well pumping; R, single well recovery. K, hydraulic conductivity] Region Near the Mine

Well ID Source* Type of 

test** Lithology K (m/s)

Total depth (m), depth to 

bedrock (m), screened 

interval (m), and notes

MMW-3

1, 3, 7 P

Andesite

1.4E-07 44.2

17.7

19.8 to 35.1

1, 3, 7 R 2.5E-08

MMW-7

1, 2, 7 S

Andesite

9.4E-08 49.1

25.6

26.2 to 49.1

1, 3, 7 P 3.1E-08

1, 3, 7 R 4.7E-09

MMW-11

1, 3, 7 P

Monzonite

1.1E-03 44.2

56.4

44.1 to 56.4

1, 3, 7 P 2.4E-06

1, 3, 7 P 3.0E-04

MMW-17B

2, 5, 7 S

Andesite and rhyolite

2.1E-05 42

26.8

37.2 to 41.8

MMW-18B

2, 5, 7 S

Granite

7.5E-06 37.5

25.3

27.1 to 36.3

MMW-19B

2, 5, 7 S

Andesite

5.4E-06 59.6

47.5

49.8 to 59.0

2, 5, 7 P 2.5E-07

2, 5, 7 R 1.8E-07

MMW-23B

2, 5, 7 S

Welded tuff

1.1E-06 30.1

3.7

20.4 to 29.6

3, 5, 7 P 3.6E-08

3, 5, 7 R 3.6E-08

K Statistics maximum 2.1E-05 Statistics do not include MMW-11 due 

to lack of drawdown during the aquifer 

test 

minimum 4.7E-09

geo mean 2.2E-07

USGS Straight Creek Research Site

SC-1B 7 S

Volcanic rock, 

likely andesite

3.7E-07

44.5

25.6

39.6 to 42.7 

7 S 4.4E-07

7 S 6.3E-07

7 S 8.0E-07

7 S 1.6E-07

SC-2B 7 S

Rhyolite

1.6E-07 26.2

16.8 (WB) and 26.2 (CB)

18.4 to 26.1

Screened in weathered (WB) and 

competent (CB) bedrock

7 S 1.1E-06

7 S 1.5E-07

7 S 9.5E-07

7 S 5.4E-08

SC-3B 7 S

Volcanic rock, 

likely andesite

6.7E-08

60.96

32.6

49.1 to 58.2 

7 S 8.3E-08

7 S 1.1E-07

7 S 1.6E-08

7 S 2.0E-08

7 S 2.7E-08

7 S 3.3E-08

7 S 1.3E-06

SC-5B 7 S

Latite

1.8E-06

128.0

86.9

103.0 to106.1 

7 S 2.2E-06

7 S 2.9E-06

7 S 1.2E-06

7 S 1.6E-06

7 S 1.9E-06

7 S 2.5E-06

K Statistics maximum 2.9E-06

minimum 1.6E-08

geo mean 3.2E-07
understanding of the complex ground-water flow system and its 

various heterogeneities.

Numerous geological features exist that may cause 

hydraulic heterogeneity in the Questa bedrock aquifer system 

(figs. 9 to 20). Several reports indicate variations in hydraulic 

properties of the bedrock aquifer system, as well as evidence 

for bedrock-derived ground-water discharge into the Red 

River and Red River alluvial aquifer in the vicinity of the 

Cabin Springs cliffs (for example, South Pass Resources, Inc., 

written commun., 1995; GSi, 1996; Slifer, 1996; Abshire, 

1998; Vail Engineering, Inc., 2000; URS, written commun., 

2001; and McCleskey and others, 2003; Kirk Vincent, written 

commun., 2005; also see fig. 9). However, few hydraulic data 

exist that robustly link bedrock hydraulic properties to specific 

bedrock structures, other heterogeneities, and the extent and 

spatial scales to which these heterogeneities behave as discrete 

and(or) continuum-type controls on ground-water flow at various spatial scales except in a fashion that is highly generalized. 

To understand the significant, relative contributions of bedrock 

ground-water flow to the alluvial aquifer and the Red River, 

the potential heterogeneities can be grouped into a hierarchical order of possibilities. The dominant and most probable 

geological features that may contribute to ground-water discharge include (1) the bedrock itself, inclusive of the various 

joint networks previously described (discussed in South Pass 

Resources, Inc., written commun., 1995a); (2) the calderamargin fault zone; and (3) the high-angle, small-displacement 

faults (figs. 8 to 19). 

Because there is no way to directly measure the groundwater flux through a sufficiently large volume and at an appropriate scale, a simple alternative approach is to use Darcy’s 

law to estimate hydraulic properties for the various geologic 

features described. Darcy’s law can be utilized in its form:

Q = KIA (1)

where Q is the volumetric flow rate [with fundamental dimensions L3

/T; L = length, T = time], K [L/T] is the bulk hydraulic 

conductivity of the geological feature of interest, I [dimensionless] is the hydraulic gradient, and A [L2

] is the specified 

cross-sectional area across which the discharge, Q, flows.

Calculations based on equation 1 assume primarily that 

the media through which flow occurs can be treated as an isotropic, homogeneous continuum where mass is conserved and 

where there are no driving forces other than gravity. Potential 

fracture aperture closure due to overburden load or tectonic 

stress is not accounted for, but the results are likely reasonable 

for the relatively near surface environment between the surface 

and underground workings of the mine in the vicinity of the 

Cabin Springs cliffs. Additionally, these computations would 

primarily represent a paleo or premining, unconfined bedrock 

ground-water flow system where effects from the present-day 

open pit and pumping from the underground mine workings 

are not accounted for. For example, South Pass Resources, 

Inc. (written commun., 1995), GSi (1996), and Abshire (1998) 

indicate that at least some component of the present-day 

bedrock ground-water flow gradient in the vicinity of Cabin 

Springs cliffs and springs is away from the river, to the north, 

due to mine-related pumping. 

However, a bedrock potentiometric-surface map constructed by URS (written commun., 2002) shows a groundwater capture zone related to dewatering of the underground 

mine workings. The mine workings are nearly a kilometer and 

a half away in a northwest direction from the Cabin Springs 

area. Directly to the north of Cabin Springs, the map shows a 

minimum of 240 meters of mapped potential hydraulic head 

above Cabin Springs. In addition, there is a large unmapped 

area with potential hydraulic head that is above and continues 

to the north of the mapped area. This area—without underground workings below it—extends an additional kilometer 

and a half and more than 300 vertical meters above Cabin 

Springs and the mapped area (URS, written commun., 2002). 

This represents a significant volume of likely saturated 

bedrock aquifer above Cabin Springs today with hydraulic 

gradients similar to those used in the theoretical computations. This gradient and volume of saturated aquifer could 

provide bedrock ground-water flow that is consistent with the 

model results reported by Kirk Vincent (written commun., 

2005) inclusive of the underground workings and the open pit. 

Although the URS (2002) potentiometric map is statistically 

derived from only a few water-level data points between the 

surface projection of the underground workings and Cabin 

Springs, the data show a significantly large mound of ground 

water between the underground mine workings and the river. 

This mound would create a ground-water divide between the 

workings and the river that could cause some component of 

flow from the north of Cabin Springs to the river as well as 

away from the river. At this time, however, the actual geometry 

of the dewatering-related capture zone is poorly constrained in 

this area. 

Although there is uncertainty regarding the effects of 

mine dewatering on the bedrock flow-system gradient, it is 

clear that ground-water discharge has interacted with the 

bedrock in the past (for example, the manganocretes in fig. 16) 

and likely continues to be expressed in the present as bedrock springs and seeps such as Cabin Springs. The computations also do not take into account the possibility of perched 

bedrock ground water that may be largely decoupled from the 

deeper bedrock flow system (see South Pass Resources, Inc., 

written commun., 1995, and Abshire, 1998). Although these 

computations are obvious simplifications, they do provide 

first-order insights into what is possible and what the contributions of the bedrock and the fault zones might be, relative to 

one another.

Several constraints provide a unique opportunity to use this 

approach for the Cabin Springs area. On the basis of U.S. Geological Survey tracer-dilution discharge data (Robert Runkel and 

Kirk Vincent, written and oral communs., 2005), an unaccountedfor, possibly bedrock-derived, ground-water inflow of 0.02 

cubic meter per second flows into the Red River alluvial aquifer 

somewhere along the Cabin Springs reach of the Red River. This 

inflow also appears to be primarily emerging along the north side 

Insights from Estimation of Bedrock, Caldera Margin, and High-Angle Fault-Zone Permeability Structures  25
26  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

of the river where there are no surficial-deposit aquifer units of 

significance (fig. 9). Vincent (written and oral communs., 2005) 

states that 0.02 cubic meter per second is a minimum value for the 

inflow. In order to cover a possible range of values in the theoretical Darcy calculations, values of volumetric flow rates (Q) of 

0.01, 0.02, and 0.05 cubic meter per second were used for multiple Darcy computations. For each value of Q, estimated and varying potential hydraulic gradients (I, four arbitrary but reasonable 

depths to water at the topographic ridge) and cross-sectional areas 

(A, five cross-sectional areas constrained by the single length of 

the reach of the Red River along which the inflow occurs and for 

an estimated, yet arbitrary but reasonable, depth of ground-water 

circulation) for the bedrock and faults were used (see figs. 9, 22, 

23, and table 5). Because there is no water table, potentiometric 

elevation, or cross-sectional area of ground-water circulation data 

available for the volume of bedrock of interest, several reasonable scenarios were chosen. Potential average topographic head 

gradients and suitable cross-sectional areas were measured using 

topographic and geologic maps and profiles (fig. 23 and table 5). 

This approach allows for multiple reasonable scenarios for the 

computation of values of hydraulic conductivity (K) to be solved 

for and compared to (1) reported bedrock aquifer-test values of K

in the Cabin Springs area (table 4) and (2) values based on model 

results from Caine and Forster (1999) using the reorganized form 

of equation 1:

K = Q/IA (2)

The results of these computations are shown in table 5. 

This table shows the input values and summary statistics for 

all of the computations for each idealized scenario. Three plots 

(see figs. 25, 27, and 28) also show the reported and computed 

values that can be compared in order to gain insight into the 

potential bedrock and fault-related flow system.

Computational Results for the Bedrock

Figure 24 shows the conceptual model set up for the 

resulting computed values of bedrock K plotted against the 

depth of the cross-sectional areas (fig. 25) with a single-line 

length (L) that intersects the ground surface. The length of the 

line corresponds to the approximate reach of the Red River 

where the suspected 0.02 cubic meter per second of bedrock 

excess inflow occurs (Kirk Vincent, written commun., 2005). 

The actual depth of circulation is unknown; however, reasonable, albeit arbitrary, estimates from 10 to 500 meters below 

the ground surface are used. Using a range of Q values of 0.01, 

0.02, and 0.05 cubic meter per second has the benefit of halving and slightly more than doubling the reported excess flow 

value. This range covers potential uncertainty in the reported 

flow due to geometry of the alluvial channel and temporal 

aspects of flow that might be caused by seasonal changes 

in base flow. Furthermore, using a range creates a scenario 

where bedrock discharges symmetrically from both the north 

and south sides of the Red River Valley to the Cabin Springs 

reach, as might be expected from a Hubbert-like model shown 

in figure 22. Values of K shown as blue points and associated 

blue lines (fig. 25) were computed for estimated minimum, 

intermediate, and maximum bedrock Qs, varying values of 

I associated with different depths to the water table at the 

watershed divide (figs. 24 and 25), and varying values of A, 

as previously discussed, using equation 2 (table 5). Reported 

hydraulic conductivities (K) from bedrock aquifer tests are 

shown as green lines of constant value.

Figure 25 shows viable solutions and intersection zones 

where the computational results intersect reported values of K. 

Intersection zones indicate a theoretically possible set of bulk 

K values for the model regions that could generate the reported 

Q values to the Red River. Figure 25 also indicates several 

attributes of the bedrock ground-water flow system. The vertical 

Figure 23. Schematic 

diagram showing an 

average topographic 

profile from the north 

divide of the Red River 

watershed to the river, 

average potential 

topographic head 

gradients for varying 

depths to water at the 

divide, and the geometry 

and parameters used for 

computations. Note: Grad 

WT refers to gradient of 

the water table, and BGS 

refers to below ground 

surface.
Table 5. Input data and summary statistics for computed and reported bedrock and fault zone hydraulic properties.

[Q, discharge; K, hydraulic conductivity; m, meters; s, seconds; BGS, below ground surface; geo mean, geometric mean; n, number of values; std dev, 

standard deviation; intersect, intersection; DDZ, distributed deformation zone; AP, aperture; μm, micrometers; RR, Red River; N, north; S, south]

Bedrock Estimates for Q Values of 0.01, 0.02, and 0.05 cubic meter per second

Note: All gradient estimates are from the northern Red River watershed divide along an approximately north-south, straight line down 

to the Red River (see figure 21 in text). All lengths in meters.

INPUT VALUES SUMMARY STATISTICS

Surface Computed Reported

length Depth Area K values K values

Gradient Average gradient scenario (m) (m) (m2) Bedrock (m/s) (m/s)**

0.205 Water table at ground surface 1,210 10 12,100 maximum 3.3E-05 2.1E-05

0.197 Water table 100 feet BGS 1,210 50 60,500 minimum 8.1E-08 4.7E-09

0.166 Water table 500 feet BGS 1,210 100 121,000 geo mean 1.2E-06 2.2E-07

0.127 Water table 1,000 feet BGS 1,210 250 302,500 n 60 13

1,210 500 605,000 std dev 6.4E-06 6.0E-06

variance 4.1E-11 2.2E-10

Caldera and High-Angle Small Fault Estimates for Q Values of 0.01, 0.02, and 0.05 cubic meter per second

INPUT VALUES SUMMARY STATISTICS

Surface Reported model 

intersect DDZ Computed K values

width depth Area K values AP & geo mean

Gradient Average gradient scenario (m) (m) (m2) Faults (m/s) (μm), (m/s)

0.349 N low-angle fault (3) segment to RR 10 10 100

Caldera

margin 

fault zones

0.201 N low-angle fault (2) segment across RR 10 50 500 maximum 4.3E-03 1,000, 4E-03

0.116 S high-angle fault (1) segment to RR 10 100 1,000 minimum 5.7E-06 500, 5E-04

See fault numbers in figure 26 as a guide. 10 250 2,500 geo mean 1.2E-04 100, 4E-06

10 500 5,000 n 45 10, 4E-09

std dev 7.8E-04

variance 6.1E-07

High-angle 

small 

fault*

0.637 N high-angle fault (4)* to RR 0.01 10 0.1 maximum 7.9E-01 100, 9.6E-03

See fault numbers in figure 26 as a guide. 0.01 50 0.5 minimum 3.1E-04

0.01 100 1 geo mean 1.2E-02

0.01 250 2.5 n 30

0.01 500 5 std dev 1.5E-01

0.1 10 1 variance 2.3E-02

0.1 50 5

0.1 100 10

0.1 250 25

0.1 500 50

*

 Modeled as a small displacement Single Fracture Fault: See Caine and Forster (1999). 

Insights from Estimation of Bedrock, Caldera Margin, and High-Angle Fault-Zone Permeability Structures  27
28 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

spread of computed K values is caused by varying gradients for 

any given cross-sectional area. Given the potentially large range 

of depth to water at the divide, the computation is relatively 

insensitive to gradient. The variation of K is a minimum of 

about one and a half orders of magnitude for the minimum to 

maximum gradients and for any single value of Q, and about 

two and a half orders of magnitude for all gradient values over 

the full range of Qs. In addition to relative insensitivity to gradient, the computation also is relatively insensitive to cross-sectional area, particularly when compared to the reported values of 

K from the bedrock-aquifer test data that span more than three 

orders of magnitude (fig. 25 and tables 4 and 5). Although there 

is a large degree of variation in the model results and data, there 

are two intersection zones between model results and aquifer 

test data that correspond to reported values of bedrock K (about 

10-5 and about 10-7 m/s, table 4). The intersecting higher value 

(Kmax) is, for comparison, similar to a well-sorted sand and is 

within the middle range for values reported for fractured crystalline rock (see Freeze and Cherry, 1979). The intersecting lower 

value (Kmean) is well within values reported for fractured crystalline rock (Freeze and Cherry, 1979). The vertical dashed green 

lines in figure 25 illustrate an example intersection zone formed 

between Kmean and model solutions for Qmin. This intersection 

zone corresponds to a circulation depth of approximately 250 to 

390 meters below the ground surface.

Finally, note that nearly all model K values range 

between the values of Kmax and Kmean reported from the aquifer 

test results (fig. 25). Thus, the model solutions indicate, given 

the stated simplifying assumptions and selected geometry of 

the models, that the bedrock alone could produce the estimated, excess ground-water discharge to the Red River surficial aquifer. As well, the K values modeled for the bedrock 

aquifer in combination with the suggested circulation depths 

could correspond to values of K and depths that are consistent with observed and reported K values for other fractured 

crystalline aquifer systems composed of a relatively high K

weathered and fractured upper zone grading to a lower K zone 

at depth (Tiedeman and others, 1998; Caine and Tomusiak, 

2003; Manning and others, 2004).

Computation and Results for the Caldera Margin 

and High-Angle Small Faults

Superimposed on the bedrock-aquifer flow system are 

sets of exceptionally different types of fault zones, as previously discussed (figs. 8 to 20). Assessment of the real hydraulic effects of fault zones on the flow system is hampered by 

the lack of any subsurface hydraulic data, such as a well that 

is known to penetrate one of these structures and that has 

Figure 24. Pseudo three-dimensional geologic and topographic map draped on a 30-meter digital elevation model with no vertical 

exaggeration showing the model setup for computations of hydraulic conductivity for the bedrock aquifer system on the north divide 

of the Red River along the Cabin Springs inflow reach. Note that the tabular blue box is meant to represent the subsurface volume 

of bedrock through which the potential ground-water discharge flows and that in the computations this box extends to the northern 

watershed divide. Z, or depth of the cross-sectional area below the surface and through which flow occurs, is varied in the computation 

and L is fixed at 1,210 meters.
carefully characterized hydraulic properties. However, the 

potential contributions of the structural heterogeneities relative 

to the bedrock alone can be estimated. This estimate can be 

accomplished in a way that is similar to the exercise completed for the bedrock; however, the model setup is different 

and is described herein. 

Figure 26 shows the conceptual model for four fault 

zones that show geological evidence of potential intersection 

with the Red River surficial aquifer in the vicinity of the unaccounted-for ground-water discharge along the Cabin Springs 

reach and region previously modeled for the bedrock alone. 

Faults labeled 1 through 3 represent possible caldera margin 

structures with one (fault 1) on the south side of the river and 

two (faults 2 and 3) on the north side of the river as mapped 

by Meyer and Leonardson (1997) (fig. 9). Although the actual 

and significant hydraulic width and depth of these structures 

are unknown, outcrop measurements at the low-angle fault 

locality show an upper-plate damage zone width of about 8 to 

10 meters. Fault 4 on the north side of the river represents one 

of the small displacement and open extensional faults (referred 

to herein as small faults) in the Cabin Springs cliffs (fig. 19), 

but the width shown in figure 26 is highly exaggerated. These 

are very narrow faults with widths from only a few centimeters 

to approximately 10 to 20 centimeters. For the computations, 

the width of the cross-sectional areas where they intersect the 

ground surface (W) and through which flow occurs was fixed 

at 10 meters for the caldera margin structures and 0.1 and 0.01 

meter for the small fault (figs. 26 and 27). The subsurface 

extent and orientation of the faults is also poorly constrained; 

thus, W was fixed and depth (Z) was varied from 10 to 500 

meters into the subsurface (fig. 26 and table 5). This approach 

results in computations for a large range of cross-sectional 

areas and fault-zone architectural parameters as presented in 

table 5. The computations also assume that the flow-source 

Figure 25. Plot of computed hydraulic conductivity (K) values relative to depth of cross-sectional areas for estimated gradients and 

values of estimated bedrock discharge (blue points and lines) to the Red River alluvial aquifer. Values of reported Ks from bedrock 

aquifer tests are shown as solid green lines and the dashed green lines illustrate a possible intersection solution zone where the model 

results are consistent with aquifer test results. Q = flow; G = gradient; min = minimum; int = intermediate; max = maximum.

Insights from Estimation of Bedrock, Caldera Margin, and High-Angle Fault-Zone Permeability Structures  29

Questa theoretical K values from bedrock Qs, Is, As

on north side of caldera margin

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

10 100 1000

DEPTH OF CROSS-SECTIONAL AREA, IN METERS

Bedrock, Qmin, G=0.205

Bedrock, Qmin, G=0.197

Bedrock, Qmin, G=0.166

Bedrock, Qmin, G=0.127

Bedrock, Qint, G=0.205

Bedrock, Qint, G=0.197

Bedrock, Qint, G=0.166

Bedrock, Qint, G=0.127

Bedrock, Qmax, G=0.205

Bedrock, Qmax, G=0.197

Bedrock, Qmax, G=0.166

Bedrock, Qmax, G=0.127

Kmax bedrock aquifer tests

Kmean bedrock aquifer tests

Kmin bedrock aquifer tests

COMPUTED K, IN METERS PER SECOND
30 Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

zone has an unknown extent and that ambient flow in the 

bedrock has no specified connection to the fault zones—the 

fault zones simply are flow regions of Q (specified by the 

excess inflow values of 0.01, 0.02, and 0.05 cubic meter per 

second as was done for the bedrock) where fault-related Ks 

were computed for specified, reasonable cross-sectional areas 

and gradients controlled by the slope and orientation of their 

mapped surface traces as shown in figure 26. 

Figure 27 shows the computational results for the fault 

zones as values of K plotted against depth of the damage zone 

or distributed deformation zone as points in lighter reds for the 

caldera structures and darker reds for the two small extensional fault scenarios with their associated lines. As with the 

bedrock, values of K were computed for estimated minimum, 

intermediate, and maximum Qs using equation 2 (table 5). 

Although there is a severe dearth of fault-related hydraulic 

conductivities from actual faults with architectural styles 

similar to those at Questa, hydraulic properties of model fault 

zones are reported by Caine and Forster (1999). The faultzone models in Caine and Forster are discrete feature models 

constructed from field-based fault-zone architectural and faultrelated fracture network data. Several of the field-based model 

scenarios in Caine and Forster (1999) are similar in style and 

size to faults at Questa, making the model results in Caine and 

Forster a reasonable first-order comparison. Finite-element 

fluid-flow simulations were run for multiple realizations of 

the fault-zone models to derive values of K for a number of 

model fault-zone architectural styles, fault-zone widths, and 

fault-related fracture-network aperture scenarios (for details 

see Caine and Forster [1999]). Values of K from Caine and 

Forster are thus used as a comparison to the results of the computational experiments presented here. These reported model 

hydraulic conductivities are shown as yellow lines of constant 

values of K in figure 27.

Like the bedrock computational results, the results for 

faults show a minimal relative sensitivity to the measured 

gradients of fault outcrop traces and circulation depths, with 

a higher range of computed Ks for the caldera structures and 

high-angle small faults as compared with bedrock (figs. 27 

and 28). Although a similar insensitivity to gradient is found 

for the two small extensional-fault scenarios and the caldera 

scenarios, it is interesting to note that the magnitude of Ks for 

either small-fault scenario is one to two orders higher than for 

any caldera scenario, in spite of the narrowness of the small 

fault scenarios that are two to three orders of magnitude narrower than the caldera structures (table 5). The primary reason 

for this is a combination of the small-fault gradient being 

larger than that of any caldera margin model and in equation 

2, area (A) is inversely proportional to K for given values of 

gradient and discharge, yet the cross-sectional areas for the 

small fault are one to four orders of magnitude lower than any 

caldera margin scenario (table 5). 

In comparison, the reported model results for bulk fault 

zone K values in a number of fault zone architectural styles 

computed by Caine and Forster (1999) largely overlap with 

the range of model K values computed here (fig. 27). This 

indicates a range of plausible fault-related ground-water flow 

scenarios over a range of reasonable circulation depths in the 

Figure 26. Pseudo three-dimensional geologic and topographic map draped on a 30-meter digital elevation model with no vertical 

exaggeration showing the model setup for computations of hydraulic conductivity associated with fault zones that likely intersect the 

Red River surficial aquifer along the Cabin Springs inflow reach. Note that tabular blue boxes are meant to represent the subsurface 

volume of fault zone through which potential ground-water discharge flows. 
vicinity of Cabin Springs. Independent of bedrock flow, the 

caldera margin structures could produce the estimated excess 

flows to the Red River surficial aquifer system with conductivities that span three orders of magnitude, largely depending 

on the mean aperture of model fault-related and open fractures 

(see Caine and Forster, 1999). Figure 27 also shows a plausible scenario where a single open fracture zone with an effective hydraulic width of 0.1 meter and conductivity of about 

10-2 m/s could produce the excess ground-water flow at Cabin 

Springs. Although fault-related fracture aperture may change 

with depth, it is likely that the effective hydraulic widths of the 

modeled fault zones are persistent to reasonable depths and are 

possibly enhanced in high-angle faults due to their subparallelism with overburden load.

As a final comparison and final important point to this 

exercise, figure 28 is the combination of computational results 

from the bedrock alone and the various fault-zone models, 

figures 25 and 27, respectively. Figure 28 shows that a DDZ 

model scenario of Caine and Forster (1999) intersects computed K values for Questa bedrock (yellow vertical dashed 

lines). The Kmax value reported from the Questa bedrock 

pumping-test data also intersects caldera margin fault scenarios (figure 28, green vertical dashed lines). The important 

point is that although there are several plausible solutions, 

there is not a unique solution as to whether the bedrock alone, 

Figure 27. Plot of computed K values relative to depth of fault damage zones for gradients and values of estimated discharge (red 

points and lines) to the Red River alluvial aquifer system. Lighter reds are results for the caldera models and darker reds are for two 

small fault scenarios for which fault width (W) estimates are shown. Values of reported model Ks from Caine and Forster (1999) for fault 

zones with architectural styles similar to those found at Questa are shown as solid yellow lines for comparative purposes (see Caine 

and Forster, 1999). Q = flow; G = gradient; min = minimum; int = intermediate; max = maximum; N = north; S = south; CSC = Cabin Springs 

Cliffs; m = meters; W = width; Ang = angle; DDZ = distributed deformation zone; SFF = single fracture fault.

Summary and Discussion of Potential Structural Controls on Bedrock Ground-Water Flow  31

Questa theoretical K values from caldera margin and

high-angle open faults for measured Qs, Is, As

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

10 100

DEPTH OF FAULT ZONE CROSS-SECTIONAL AREA, IN METERS

Low-angle caldera margin fault N, Qmin

High-angle caldera margin fault N, Qmin

Low-angle caldera margin fault S, Qmin

Low-angle caldera margin fault N, Qint

High-angle caldera margin fault N, Qint

Low-angle caldera margin fault S, Qint

Low-angle caldera margin fault N, Qmax

High-angle caldera margin fault N, Qmax

Low-angle caldera margin fault S, Qmax

K 1,000 micron DDZ

K 500 micron DDZ

K 100 micron DDZ

CSC high-angle open fault, Qmin, W = 0.01 m

CSC high-angle open fault, Qint, W = 0.01 m

CSC high-angle open fault, Qmax, W = 0.01 m

CSC high-angle open fault, Qmin, W = 0.1 m

CSC high-angle open fault, Qint, W = 0.1 m

CSC high-angle open fault, Qmax, W = 0.1 m

K 100 micron SFF

CO

MPU

TED

K, IN

METERS PER SECO

N

D
32  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

one or more of the fault zone scenarios, or some combination 

are producing the excess ground-water inflow to the Red River 

alluvial aquifer.

Summary and Discussion of Potential 

Structural Controls on Bedrock 

Ground-Water Flow

The rocks exposed along the southern margin of the 

Questa caldera have undergone significant brittle deformation that can be characterized as structural heterogeneities that 

may have various potential effects on the bedrock groundwater flow system. These structures include (1) bedrock joint 

networks, (2) high-angle, small displacement faults, (3) the 

caldera-margin fault zone, (4) stockwork and other vein 

networks, (5) lithologic contacts, and (6) the range-front fault 

system. Additionally, it is important to note that the rocks at 

Questa reveal a unique exposure of a deeply eroded caldera 

system, and the extent to which the various geological structures represent other caldera fracture-dominated flow systems, 

particularly the caldera margin and its structural elevation, is 

unknown.

All joint-network properties described herein for all 

observed Questa bedrock lithologies indicate a system that is 

composed of high-intensity, highly interconnected, multiple 

Figure 28. Combined plot (plots 25 and 27 above) of computed K values relative to geological feature cross-sectional area depth for the 

bedrock alone and for fault damage or distributed deformation zones. Note that the values of reported bedrock aquifer Ks from Questa 

pumping tests and reported model Ks from Caine and Forster (1999) shown as solid horizontal lines have overlapping intersection 

zones depicted as vertical dashed lines, indicating that there is not a unique solution as to which one of these geological elements 

is producing the estimated discharge to the Red River. Q = flow; G = gradient; min = minimum; int = intermediate; max = maximum; N 

= north; S = south; CSC = Cabin Springs Cliffs; m = meters; W = width; Ang = angle; DDZ = distributed deformation zone; SFF = single 

fracture fault.

Questa theoretical K values from bedrock, caldera margin

and high-angle open fault Qs, Is, As

1E-09

1E-07

1E-05

1E-03

1E-01

1E+01

10 100 1000

Bedrock, Qmin, G=0.205

Bedrock, Qmin, G=0.197

Bedrock, Qmin, G=0.166

Bedrock, Qmin, G=0.127

Bedrock, Qint, G=0.205

Bedrock, Qint, G=0.197

Bedrock, Qint, G=0.166

Bedrock, Qint, G=0.127

Bedrock, Qmax, G=0.205

Bedrock, Qmax, G=0.197

Bedrock, Qmax, G=0.166

Bedrock, Qmax, G=0.127

Kmax bedrock aquifer tests

Kmean bedrock aquifer tests

Kmin bedrock aquifer tests

Low-angle caldera margin fault N, Qmin

High-angle caldera margin fault N, Qmin

Low-angle caldera margin fault S, Qmin

Low-angle caldera margin fault N, Qint

High-angle caldera margin fault N, Qint

Low-angle caldera margin fault S, Qint

Low-angle caldera margin fault N, Qmax

High-angle caldera margin fault N, Qmax

Low-angle caldera margin fault S, Qmax

K 1,000 micron DDZ

K 500 micron DDZ

K 100 micron DDZ

CSC high-angle open fault, Qmin, W = 0.01 m

CSC high-angle open fault, Qint, W = 0.01 m

CSC high-angle open fault, Qmax, W = 0.01 m

CSC high-angle open fault, Qmin, W = 0.1 m

CSC high-angle open fault, Qint, W = 0.1 m

CSC high-angle open fault, Qmax, W = 0.1 m

K 100 micron SFF

COMPUTED K, IN METERS PER SECOND

DEPTH OF CROSS-SECTIONAL AREA, IN METERS
orientation, generally small aperture joints. These joint networks 

can be thought of as the “background permeability structure” 

of the bedrock. This background structure may be relatively 

homogeneous and nearly isotropic, albeit low permeability, 

with respect to bulk hydraulic properties of each bedrock 

lithology at the well field to caldera scale (based on numerical 

modeling in Caine and Tomusiak, 2003, for example). These 

small-aperture joint networks also may provide bulk storage and 

geochemical reaction space from which larger aperture joints 

and faults obtain ground-water discharge, particularly when 

the aquifer is stressed. Iron oxide staining restricted to several 

joints in a highly connected joint network (see fig. 7) may be 

representative of the “plumbing,” and the discrete nature of 

flow in general, in the shallow portion of the bedrock aquifer. 

Various discrete features such as an unusually wide aperture 

joint or open-fault zone that cut this “background permeability 

structure,” such as those in the rhyolites outside of the ore body, 

may be the most important features in terms of heterogeneity 

and anisotropy and thus solute transport within the upper few 

hundred meters of the bedrock aquifer and ground-water flow 

system. 

There are two dominant styles and orientations of majorfault zones: the low-angle, east-west-striking composite 

deformation zone of the caldera margin and the high-angle 

north-south-striking composite and distributed deformation 

zones, both of which cut the low-angle CDZ. The DDZs in 

particular show extensive internal-opening mode distributed 

deformation (figs. 3, 9, and 18). Within the lower Red River 

watershed, the topographic gradient and assumed hydraulic 

head gradient are toward the river, exclusive of potential local 

reversals of the hydraulic gradient due to mine dewatering 

(figs. 22 and 29). On the basis of the open nature of many of 

the high-angle extensional faults and the assumed potential 

hydraulic gradient, whether affected by mine dewatering 

(potential present-day case) or not (premining case), it can be 

inferred that there is great potential for there to be significant 

but highly discrete, and possibly randomly placed, north-tosouth anisotropy caused by the open and crosscutting, highangle, small extensional faults. Further, the intersections of 

the high-angle structures with any subhorizontal structure, 

whether the subhorizontal structure has a low-permeability 

clay-rich core or not, and particularly if they have an extensively fractured damage zone, may also cause north-to-south 

anisotropy in the ground-water flow system (see Caine and 

Forster, 1999). 

The presence of brecciated iron and manganese oxide 

clasts in the high-angle CDZ fault-core rocks suggests an 

ancient, synmineralization origin for these elemental constituents. If the processes that formed this CDZ also formed the 

other mapped CDZs in this area (fig. 8), then these fault zones 

also may collectively be significant and localized, long-lived 

sources of bedrock-derived iron and manganese loading to 

bedrock ground water. Manganese dendrite surface coatings 

(not veins or breccias) in the fault cores of the high-angle 

Figure 29. Fifteen-meter resolution natural color satellite image draped on a USGS digital elevation model with no vertical 

exaggeration. The topographic and geologic margin of the Questa caldera faces the reader. Note the extreme topographic gradient 

down toward the Red River; many of the high-angle extensional faults observed in the Cabin Spring cliffs area trend north-south and 

parallel this gradient.

Summary and Discussion of Potential Structural Controls on Bedrock Ground-Water Flow  33
34  Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow

DDZs below the CDZs were observed, as were manganocretes, along the banks of the Red River below these faults. 

This relationship attests to the high probability of long-lived, 

downgradient bedrock ground-water flow from the upper 

CDZs, through the DDZs, to discharge at the banks of the Red 

River, ultimately forming the manganocretes.

Although many of the fault zones in the pit ore body are 

filled with molybdenite, open damage-zone-related fractures may 

exist that are potential conductors of ground water. These structures also may be connected to the caldera margin and may be a 

dead end for anisotropic flow or may be a conduit that is capable 

of transporting mass from high levels of the ground-water flow 

system both vertically and laterally to lower parts of the groundwater and surface-water systems. The range-bounding fault zone 

may have many open brittle structures in its bedrock footwall 

block and relatively highly connected, combined conduit-barrier 

flow properties—the range-bounding fault is probably not much 

of a contributor to the ground-water flow system until the region 

near the mountain front (figs. 1 and 3). Veins are likely to be lowpermeability features with respect to their host rock unless they 

are jointed, and the extensive stockwork vein systems may act to 

uniformly lower the bulk permeability of the bedrock at the well 

field scale where the systems exist in the subsurface. Planar vein 

swarms also may locally baffle or channel flow due to their presumed low permeability relative to their host rock, unless they are 

jointed, in which case they may cause no appreciable anisotropy.

On the basis of the descriptions of the caldera margin, the 

caldera margin might be a good candidate to supply bedrock 

ground-water discharge to the Red River. As mapped in figure 1, 

the caldera margin crosses the Red River in three places. When 

the positions of these crossings are compared with base-flow 

tracer-dilution study data that indicate the instream locations of 

surface water compared to ground-water discharges, only the 

crossing at Cabin Springs shows significant potential groundwater discharge to the river (McCleskey and others, 2003). 

The sources of such ground-water discharges are not, however, 

uniquely determined from the tracer study alone. For example, 

such discharge could come from either a high-permeability 

layer in substream channel alluvial materials, a zone of relatively high intensity fracturing, or a fault zone in the bedrock. 

The constraints on values of Q and work by Kirk Vincent (written commun., 2005) along the Cabin Springs reach represents 

a unique case where an unaccounted-for bedrock ground-water 

discharge further brings into question whether or not the caldera 

margin could produce this discharge or if it could be produced 

from the background permeability structure of the bedrock 

alone.

The spatial variability of the caldera-margin permeability 

structure is highly speculative, particularly with depth. If the 

low-angle fault exposure (figs. 8, 9, and 11) is a permeability 

structure analog for the entire caldera margin, it could have the 

architecture of a combined conduit-barrier. However, a number 

of processes could alter this permeability structure in the subsurface even if the architecture is the same as that at the surface. Of 

course, this structure may be different at depth and at different 

locations. However, if the margin is everywhere a fault zone that 

formed under the same conditions and processes, it could be 

similar to what is seen at Cabin Springs, particularly with regard 

to the extremely high intensity fracturing in its damage zone. 

Some of the processes and their possible combinations that 

could alter the permeability structure and hydrologic influence 

of the caldera margin include (1) decreased fracture aperture 

and closure (or opening, depending on fracture orientation as 

discussed herein) with depth due to overburden load (Davis and 

Turk, 1964; Snow, 1970 and 1972), (2) proximity of the caldera 

margin to fluids and mineral precipitates in them as well as 

various sources of heat and chemical/rock reactions capable of 

sealing the fault zone at depth, (3) variations in the lithologies 

that are cut by the margin structure and their varying responses 

to brittle deformation during the evolution of the zone, and 

(4) variations in the present-day hydrologic regime with respect 

to the location of the caldera margin (for example, topography 

and head gradients, precipitation, natural and anthropogenic 

discharge, and recharge).

In numerous regions throughout the United States and 

elsewhere, where there is low-permeability fractured crystalline bedrock underlying higher permeability surficial deposits and weathered bedrock, the bedrock is not considered a 

significant ground-water source (Davis and Turk, 1964; Caine 

and Tomusiak, 2003). Simple calculations presented here, 

however, show that the bedrock permeability structure alone, 

when treated as a continuum, could deliver inferred groundwater discharge to the Red River alluvial aquifer in the Cabin 

Springs reach of the Red River.

The calculations also show that if the architecture of the 

entire caldera margin is similar to that observed at the low-angle 

fault locality and if it is not sealed by mineral precipitation 

where it cannot be observed, the margin also could produce 

the significant and unaccounted-for discharge along the Cabin 

Springs reach. The calculations further show that an individual 

high-angle, open extensional fault that intersects the Red River 

in the vicinity of Cabin Springs and is parallel to the valley-side 

topographic head gradient also could produce the unaccountedfor discharge. Therefore, the calculations and all of the data 

available show that under the stated assumptions it is plausible 

that the bedrock alone, and(or) various fault zones that cut it, 

could produce bedrock ground-water discharge to the Red River 

alluvial aquifer. However, there is not a unique solution to the 

source of the unaccounted-for Cabin Springs ground-water 

discharge or probably for any similar discharges. Thus, geological observations and simple calculation are not hydraulic reality, 

and inferences made, such as the ones presented here, must be 

used with caution. Their value is that they may provide a set 

of testable hypotheses of what might be theoretically possible 

as well as geological constraints to explain certain hydraulic 

responses during borehole logging, other various aquifer tests, 

and possibly stream tracer-dilution studies.

Although there has been much work done in the Cabin 

Springs area, there remains a dearth of data and large uncertainties associated with understanding the bedrock flow system 

and its inherent structural and hydrogeological heterogeneities. 

Should one desire to quantify the actual permeability of a specific 
volume of bedrock or geologic structure, such as the caldera 

margin, and its actual discharge contribution to an overlying 

surficial aquifer, there are several research and data-collection 

approaches that alone or in combination could provide greater 

insight. These approaches might include (1) further detailed, 

site-specific geological observations; (2) high-resolution seismic data acquisition; (3) drilling and coring new boreholes that 

are designed specifically to sample the bedrock or a discrete 

structure of interest that would include angled or horizontal 

holes if necessary; (4) detailed geophysical logging with optical and acoustic televiewers and heat pulse or electromagnetic 

flowmeters that would be used to identify, isolate, and sample 

flow specifically from the discrete bedrock zones or structures; 

(5) detailed, site-specific streamflow tracer-dilution and seepage 

studies that target specific geological features at various scales 

by using fault-rock geochemical, mineralogical, and paragenetic 

studies (for example, above and below the caldera margin where 

it crosses the Red River, coupled with multiple geochemical and 

thermal samplings of water from concurrent aquifer hydraulic 

testing in the well-characterized boreholes and nearby spring and 

seeps); and (6) dye and other tracer-injection studies utilizing the 

boreholes, stream, springs, and seeps. These data should also be 

integrated with detailed structural, petrological, geochemical, and 

petrophysical characterization of recovered core. Discrete interval 

and integrated borehole hydraulic testing coupled with waterlevel monitoring in well-characterized boreholes penetrating 

bedrock or discrete structures are likely to provide (1) important 

constraints on the potential contributions of bedrock ground water 

to acid and trace-metal loading and transport; and (2) constraints 

on how the bedrock might interact with surficial deposits, the 

open-pit mine, and underground mine workings when they are 

ultimately abandoned.

Acknowledgments

This work was funded by the U.S. Geological Survey, 

Water Resources Discipline, and done in cooperation with 

the New Mexico Environment Department. Other support 

came from the U.S. Geological Survey, Geologic Discipline, Mineral Resources Program. Permission to access 

the mine site was granted by MolyCorp, Inc., Questa, New 

Mexico. Jeff Meyer and Bruce Walker graciously showed us 

field localities and freely granted use of various maps and 

data. Reviews by Don Sweetkind and D. Kirk Nordstrom 

improved this manuscript as did discussions with Geoff 

Plumlee, Andy Manning, Philip Verplanck, and Kirk Vincent. 

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By Jonathan Saul Caine—Questa Investigation 18—Brittle Structures and Their Potential Influence on Bedrock Ground-Water Flow—USGS Professional Paper 1729

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