UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

Structural Evolution of a Grand Canyon Breccia Pipe:

The Ridenour Copper-Vanadium-Uranium Mine, Hualapai Indian Reservation,

Coconino County, Arizona

by

1 1 9 Earl R. Verbeek , Marilyn A. Grout , and Bradley S. Van Gosen

Open-File Report 88-006

Prepared in cooperation with the 

Hualapai Indian Tribe

This report is preliminary and has not been reviewed for conformity with U.S. 

Geological Survey editorial standards and stratigraphic nomenclature. Use of 

trade names is for descriptive purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

U.S. Geological Survey 

Box 25046, MS 913 

Denver Federal Center 

Denver, CO 80225

"U.S. Geological Survey 

Box 25046, MS 905 

Denver Federal Center 

Denver, CO 80225

1988
CONTENTS

Page

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

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

Terminology......................................................... 4

Units of measurement................................................ 5

Mine location and access............................................ 5

Stratigraphic setting............................................... 6

Structural setting.................................................. 7

General character of breccia pipes...................r................... 8

Solution-collapse origin............................................ 8

Dimensions.......................................................... 8

Breccia clasts and ring fractures................................... 9

Mineralization and alteration....................................... 9

Fracture network near Ridenour mine...................................... 11

Methods of study.................................................... 12

Description of joint sets........................................... 14

Comparison of field and photogeologic studies....................... 22

Times and depths of jointing........................................ 24

A comment about joint styles........................................ 26

Mineralization and alteration along joints.......................... 27

Reactivation of joints.............................................. 30

Stresses inferred from joint sets................................... 31

Summary and conclusions............................................. 34

Geology of the Ridenour mine............................................. 34

General configuration of pipe and mine.............................. 34

Production history.................................................. 36

Host rocks near mine................................................ 37

Structure of the pipe core.......................................... 37

Structure of the ring-fracture zone................................. 39

Alteration of pipe fill and surrounding country rock................ 48

Pre-ore deposition of carbonate and sulfate minerals................ 49

Ore mineralization.................................................. 51

Joint network in mine............................................... 54

Supergene minerals in J1-J5 joints.................................. 58

Chronology of pipe development...................................... 59

Discussion............................................................... 62

Paleohydrologic implications........................................ 62

Structural control of breccia pipes................................. 64

Implications for mining of breccia pipes............................ 66

Acknowledgments.......................................................... 68

References cited......................................................... 69

Appendix A Measured Stratigraphic section near Ridenour mine............ 74
STRUCTURAL EVOLUTION OF A GRAND CANYON BRECCIA PIPE:

THE RIDENOUR COPPER-VANADIUM-URANIUM MINE, HUALAPAI INDIAN

RESERVATION, COCONINO COUNTY, ARIZONA

Earl R. Verbeek, Marilyn A. Grout, and Bradley S. Van Gosen

U.S. Geological Survey, M.S. 913, Denver Federal Center

Denver, CO 80225

ABSTRACT

The Ridenour Cu-V-U deposit, in western Coconino County, Arizona, was mined for 

copper intermittently from about 1887 to about 1916, and for uranium and vanadium in 

the early 1960's. The ore occurs within an arcuate belt of fractures in flat-lying beds of 

Permian sandstone along the periphery of a solution-collapse breccia pipe. The pipe 

formed by upward stoping into the Esplanade Sandstone, the host formation, from initial 

solution caverns of probable Late Mississippian age in the underlying Redwall Limestone. The time of stoping is not known with certainty, but U-Pb age determinations on 

ore from similar, nearby pipes show that the main phase of uranium mineralization 

occurred 200-220 Ma (Middle to Late Triassic). The ore is now oxidized, but studies in 

other pipes where the primary ore assemblage is better preserved show that copper was 

deposited mostly as sulfides and sulfosalts, and the uranium as uraninite. The original 

host minerals for vanadium in the Ridenour pipe are not yet known.

Strata bordering the Ridenour pipe are cut by a well-developed set of "ring 

fractures" that formed during stoping, and by five regional sets of joints. The ring 

fractures strike everywhere parallel to the pipe boundary, dip moderately away from it, 

and occupy a zone ^0-130 ft (12-38 m) wide around the brecciated pipe core. These were 

the only fractures in existence at the time of mineralization. Permeability conduits 

during ore deposition, then, were limited to the breccia column of the pipe proper and to 

the open ring fractures surrounding it. Most ore from the Ridenour mine was produced 

from long, narrow stopes that followed dense vein arrays along the most heavily mineralized portions of the ring-fracture zone. The later joints contain only sparse films of 

secondary minerals, chiefly malachite and azurite, that formed upon late Cenozoic 

oxidation of the primary ore minerals and minor redistribution by ground water of metals 

derived from them.

INTRODUCTION

The Ridenour copper-vanadium-uranium deposit occurs within an arcuate belt of 

fractures bordering a partially eroded, solution-collapse breccia pipe on the Hualapai 

Indian Reservation in western Coconino County, Arizona. This pipe is one of hundreds of 

such features hosted by Paleozoic sedimentary rocks in the Grand Canyon region of the 

southwestern Colorado Plateau (fig. 1). The pipes initiated by local collapse of structurally unstable portions of large solution caverns within the underlying Mississippian 

Redwall Limestone, which, by the close of Mississippian time, had already been riddled 

by an extensive cave system (Billingsley, 1986). Continued collapse resulted in the 

upward propagation of nearly vertical, pipelike conduits clogged with collapse breccia 

derived from overlying strata. Locally the pipes penetrate the entire Paleozoic section 

above the Redwall and extend into the overlying Triassic Moenkopi and Chinle 

Formations (fig. 2), a vertical distance of 2,000 ft (600 m) or more. Some of the pipes 

and bordering country rock are conspicuously mineralized at the surface (Wenrich, 1985; 

Sutphin, 1986), and a few (nos. 2, ^, 5, 8, and 9 of fig. 1) were worked for copper before 

the turn of the century (Billingsley, 197^). All were small operations, and most were
109

ARIZONA STRIP

PLATEAU\ PLATEAU 

&,

HUALAPAI 

PLATEAU COLORADO 

PLATEAU

Hualapai Indian 

Reservation

BASIN AND RANGE PROVINCE

34

Figure 1. Index map of northern Arizona showing location of features discussed in text; 

modified from Wenrich (1985, 1986a). Numbered mines are as follows:

1 Copper House

2 Copper Mountain

3 Cunningham

4 Grand Gulch

5 Grandview

6 Hack Canyon

7 Old Bonnie

8 Orphan

9 Ridenour

10 Riverview

11 Savanic

12 Snyder

13 Pigeon

14 Kanab North

15 Canyon

16 Pinenut

17 Hermit

soon abandoned. Interest in the Grand Canyon region was rekindled in the 1950's by the 

discovery of rich uranium ore in some pipes. Though the deposits generally were small 

and far from the nearest mill, and some were difficult to mine due to their location on 

sheer canyon walls, the abnormally high grade of their ores attracted sufficient attention 

that several mines were put into production. The most productive of these, the Orphan 

mine (pipe no. 8 on fig. 1), yielded 495,107 tons of uranium ore at an average grade of 

0.43% U^Og, 6.68 million pounds of copper, 3,283 pounds of vanadium oxide, and 107,000 

troy ounces of silver between 1956 and 1969 (Chenoweth, 1986). During this period the 

number of known pipes was still small.

A third cycle of interest in breccia-pipe deposits of the Grand Canyon region began 

in the mid-1970's, when it became apparent that their numbers are far greater than 

previously thought (Wenrich-Verbeek and others, 1980). The number of suspected pipes 

on the Hualapai Reservation alone now stands at more than 900 (Wenrich, Billingsley, and 

Van Gosen, 1986), of which about 100 have been confirmed. Though only a fraction of

*The corresponding metric equivalents are 449,154 metric tons (U ore), 3.03 million kg 

(Cu), 1,489 kg (V 205 ), and 3,328 kg (Ag).
-Diameter of surface depressionO

CO 

CO

<

DC

IAverage thickness

or range in thickness

(in feet)

Chinle 

Formation

Petrified Forest 

Member

Shinarump Member

Moenkopi 

Formation

«a

 ^^^^ ^ -.^ :^L^'^: 4 °° ' ' 2 °° z^-0m±^m: -^- .:.-. - :.-.-^. '.. ;mTT.".- Coconino 

Plateau

Harrisburg Gypsiferous 

Member

Fossil Mountain Member

Woods Ranch Member 

Toroweap

Formation Brady Canyon Member 

Seligman Member

Coconino Sandstone

Hermit Shale 500

Esplanade 

Platform ,

Esplanade Sandstone

Ridenour mine

Wescogame Formation

Watanomigi Formation

Surprise Canyon

Formation 

(Mississippian/Pennsylvanian(?>)

Temple Butte Limestone

Figure 2. Generalized vertical section through a solution-collapse breccia pipe of the 

Grand Canyon region. Thicknesses of units shown are typical of those observed on the 

Hualapai lands.
these show surface indications of mineralization, the potential for extraordinarily high 

ore grades has encouraged continued exploration for new pipes and drilling of the most 

promising examples. In some pipes the ore averages 0.65% or more UjOg (Mathisen, 

1987) and is associated with potentially economic concentrations of Ag, Pb, Zn, Cu, Co, 

and Ni as byproducts (Wenrich and Silberman, 1984; Wenrich, Billingsley, and Van Gosen, 

1986). By mid-1987, one company was already operating six breccia-pipe mines in the 

Grand Canyon region and had requested permits to open a seventh (Mathisen, 1987).

This report focuses on the structural evolution of a particularly well exposed, 

mineralized breccia pipe. Topics addressed include the development of the mineralized 

fracture zone peripheral to the pipe, the influence of pipe structure on alteration and 

mineralization, and the timing of pipe formation and ore deposition relative to faulting 

and jointing of the host rocks.

TERMINOLOGY

The literature on structural and economic geology is replete with multiple and 

often vague or curious definitions of the same term. Even such common terms as fault 

and joint seem to invite confusion, and thus some explanation of terminology used in this 

report is warranted. For the most part our usage conforms to that most commonly 

employed in recent literature on the Arizona breccia-pipe deposits.

Breccia pipe- One specific type of collapse feature (also defined below). Breccia pipes in 

the Grand Canyon region are roughly cylindrical, nearly vertical columns of broken rock, 

within which the clasts at any given level occur below their normal stratigraphic 

positions. Unlike breccia pipes from many other localities, the Grand Canyon examples 

are nonvolcanic and formed by progressive collapse of strata into solution caverns that 

formed in underlying carbonate rocks. The term as used in recent literature on the 

Grand Canyon area refers only to those breccia columns known or suspected to bottom in 

the Redwall Limestone, the principal cave-forming unit of the region.

Collapse feature, solution-collapse feature- Any feature formed by foundering of strata 

into an underlying opening created (for the Grand Canyon examples) by the removal in 

solution of carbonate rocks or gypsum. Included in this definition are such diverse 

features as (1) sinkholes in the Kaibab Limestone capping the Coconino and Marble 

Plateaus (Sutphin and Wenrich, 1988; Sutphin, 1986; Wenrich, Billingsley, and Hun toon, 

1986), (2) structural basins formed by local dissolution of gypsiferous strata in the 

Toroweap Formation and Kaibab Limestone (Billingsley and others, 1986; Wenrich, 

Billingsley, and Hun toon, 1986), and (3) breccia pipes as defined above. The word 

collapse does not necessarily imply the sudden falling of rock into large subterranean

voids; for some structures an origin by episodic subsidence into openings created gradually by solution seems more likely.

Fracture- a general term for any surface within a rock across which cohesion has been 

lost (or had been lost, if healed by mineralization); a break. The term thus includes 

faults, joints, and fissures, all defined below.

Fault- a fracture in which the dominant component of movement has been shear, 

parallel to the fracture walls.

Fissure- an open fracture; one in which the dominant component of movement has 

been extension normal to the fracture walls, resulting in their separation.

Joint- a fracture associated with no significant displacement in any direction. 

What constitutes "significant" is arbitrary and more a question of semantics than science, 

but most workers would probably call any fracture associated with readily visible offset 

at the outcrop scale a fault jratherJtian a joint^ We retain tha.^conve^ntionjiere and
restrict usage of joint to refer only to fractures whose walls have not moved more than 

0.4 inch (1.0 cm) relative to each other in any direction. Upon subsequent shear or 

extension a joint can become a fault or fissure, respectively; these are reactivated joints. 

Joint, as defined here, is strictly a geometric term, in keeping with common 

usage. Note, however, that the common definition of joint as "a surface of fracture or 

parting in a rock, without displacement" (Bates and Jackson, 1980) is nonsensical: if the 

rock along one joint wall did not move in any direction relative to rock along the 

opposing wall, the crack between them could not exist.

Fracture length, fracture height- in horizontal beds, the dimensions of a fracture as 

measured parallel to its strike and dip lines, respectively. Few fractures are exposed in 

their entirety, however, so true lengths and heights commonly are not measurable. Some 

dimensions cited in this paper refer to exposed (partial) heights and lengths and are so 

termed where appropriate.

Ring fracture, ring-fracture zone, annular ring- related terms with at least two contrasting meanings. An annular zone of concentric, outward-dipping fractures, often highly 

mineralized, occurs within country rock along the periphery of some breccia pipes. To 

these fractures some workers apply the term ring-fracture zone or, less accurately, ring 

fracture. Our usage conforms to this definition. Others, however, use ring fracture in a 

quite different sense, to refer to the contact between brecciated rock of the pipe proper 

and the country rock surrounding it. The term "annular ring" is redundant and not 

recommended.

UNITS OF MEASUREMENT

Nearly all mining reports on mineralized breccia pipes of the Grand Canyon region 

employ English units as the basis of measurement, and topographic maps of the area still 

show elevations and contour lines in feet rather than meters. For these reasons, all 

measurements in this paper are stated first in English units followed by their metric 

equivalents in parentheses.

Wherever a number is used only in an approximate sense, as in such statements as 

"Diameters of most breccia pipes range from about 100 to 400 ft", the given metric 

equivalent is approximate also and has been rounded to the nearest simple number ("30 to 

120 m"). Exact metric equivalents are provided for values based on actual measurements 

reported to higher standards of accuracy.

MINE LOCATION AND ACCESS

The Ridenour mine (fig. 3) is located in the northeastern part of the Hualapai Indian 

Reservation in sec. 6, T31N, R8W, high on the west wall of an unnamed canyon at an 

elevation of 4,480 ft (1,365 m). The canyon drains northwestward into the Colorado 

River, 2.0 mi (3.3 km) distant and several thousand feet lower. Access to the mine from 

Peach Springs, the nearest town, is by paved road 30 mi (48 km) to Frazier Wells (elev. 

6,058 ft, or 1,846 m), and thence by generally good dirt roads that ascend northward 

across the Coconino Plateau and then descend within stream valleys through the Aubrey 

Cliffs. From there, other roads lead to the rim of the Grand Canyon at an elevation of 

6,320 ft (1,926 m). A steep 4-wheel-drive road blasted into the cliff face descends into a 

tributary valley to the Colorado River, where a series of unimproved dirt roads leads 

eventually to the Ridenour mine. Total distance from Peach Springs to the mine is 59 mi 

(94 km) by road.
36° 00' 

Figure 3. Map of Hualapai Indian Reservation, showing major structural features and 

localities discussed in text. HFZ = Hurricane fault zone; TFZ = Toroweap fault 

zone. Modified from Wenrich (1986a).

STRATIGRAPHIC SETTING

The stratigraphic level of the Ridenour mine was said by Miller (1954) to be about 

the middle of the (then unnamed) Esplanade Sandstone of early Permian age, but newer 

mapping by Wenrich, Billingsley, and Huntoon (1986) shows it to be near the base of this 

formation. The Esplanade Sandstone the uppermost formation of the Supai Groupaverages about 450 ft (135 m) thick in this area and consists predominantly of pale red to 

reddish orange, cross-stratified, fine to medium-grained sandstone with subordinate 

siltstone and limestone (Wenrich, Billingsley, and Huntoon, 1986).________________
The Esplanade Sandstone caps a minor plateau surface (the Esplanade Platform) for 

several square miles north and southwest of the mine, at a general elevation of about 

4,800 ft (1,460 m), and is well exposed within stream valleys and canyons cut into that 

surface. Farther south and east, toward the rim of the Grand Canyon, successively 

younger strata are exposed along prominent cliff faces ascending to a broad plateau 

capped by the Kaibab Limestone at elevations of 6,300-6,600 ft (1,920-2,010 m). To the 

north and west lies the Inner Gorge of the Grand Canyon, whose south rim is less than a 

mile distant from the mine. Stepped cliffs lead downward through the Paleozoic section 

to the Colorado River, where, at elevations of about 1,600 ft (490 m), Precambrian 

metamorphic rocks of the Vishnu Group crop out sparingly. Nearly the entire succession 

of Paleozoic strata, about 4,800 ft (1,460 m) thick, is exposed in this area.

STRUCTURAL SETTING

Dominant structures of the region are the Hurricane and Toroweap fault zones, 

both coincident for much of their extent with monoclines of the same name (fig. 3). The 

monoclines, as expressed at the level of the Esplanade Sandstone, are gentle ramplike 

features of overall northerly trend and with gently tilted (5°-10°) limbs averaging 1-1.5 

mi (1.6-2.5 km) in width (Wenrich, Billingsley, and Huntoon, 1986). Each monocline warps 

the strata downward to the east and is a relatively young feature of Laramide age 

(Huntoon, 1981). The faults with which they are associated, however, are of Precambrian 

ancestry and have a complex and lengthy history of movement, often in opposing senses. 

A brief listing of the major Phanerozoic events of the region, derived from Wenrich, 

Billingsley, and Huntoon (1986) and references cited therein, is as follows:

7D Regional subsidence and accumulation, from Cambrian through Cretaceous 

time, of 8,000 to 13,000 ft (2,440 to 3,960 m) of sediment. These sediments were 

deposited on deeply eroded metamorphic rocks (the Vishnu Group) that had already been 

offset, in Precambrian time, along numerous normal faults.

____(2) Laramide (Late Cretaceojjs through Eocene) crustal compression. During this 

time some of the old Precambrian normal faults those most favorably oriented^to 

accomodate subhorizontal, ENE-directed crustal compression were reactivated as 

reverse faults. Ramplike folds, among them the Hurricane and Toroweap monoclines, 

developed above the principal zones of reverse movement. Structural relief along most 

of the length of both monoclines ranges from 300-900 ft (90-270 m), east side down.

Regional uplift and unloading by erosion began during the Laramide movements and 

continued through the whole of Cenozoic time. Post-Laramide faults and joints, then, 

probably developed at shallower crustal levels (lesser confining pressures) than the 

Laramide monoclines.

(3) Recurrent Miocene, and later, normal faulting accompanied by continued uplift 

and erosion. During the Miocene epoch the southwestern margin of the Colorado Plateau 

became structurally and topographically distinct from the Basin and Range Province 

(Young and Brennan, 1974). Normal slip, west side down, occurred along some of the 

same faults that earlier, in Laramide time, had accomodated reverse slip to form the 

major monoclines of the region. The structurally high, west limbs of the monoclines now 

were faulted downward to positions below their eastern limbs. As crustal extension 

continued through the late Cenozoic, new faults striking within 20° of due north formed 

in progressively greater numbers within the previously little-faulted areas between the 

major fault zones of Precambrian ancestry. Today much of the region is riddled with 

minor faults and grabens, and faulted Quaternary deposits suggest that crustal extension 

is continuing.
GENERAL CHARACTER OF BRECCIA PIPES 

SOLUTION-COLLAPSE ORIGIN

Horizontal to gently dipping Paleozoic and Mesozoic strata of the Grand Canyon 

region are penetrated by hundreds of pipelike bodies of breccia that are circular to 

broadly elliptical in plan view and whose axes are vertical, or nearly so (fig. 2). These 

structures are considered by most recent workers (Bowles, 1965, 1977; Gornitz and Kerr, 

1970; Wenrich-Verbeek and others, 1980; Wenrich, 1985; Sutphin, 1986) to have formed by 

progressive collapse and consequent upward stoping of gravitationally unstable portions 

of large solution caverns in the Mississippian Redwall Limestone. The stoping process in 

many pipes likely was aided by the gradual breakdown of carbonate-cemented sandstone 

clasts, derived from overlying Pennsylvanian and Permian strata, into their component 

grains through dissolution of carbonate cement. Removal of some of this material, both 

in solution and by downward sifting of loose grains through the pipe and flushing through 

the Redwall cavern system, created the additional room necessary for continued collapse 

and upward propagation of pipes through, in some cases, more than 2,000 ft (600 m) of 

section. The pipes thus are sedimentary features, unlike the mineralized pipes of many 

other areas that owe their origin to explosive volcanism. Evidence for a solutioncollapse origin of the Grand Canyon pipes is voluminous and includes the following 

facts: (1) With the local exception of several enigmatic breccia bodies in Cambrian rocks 

(Wenrich, Billingsley, and Huntoon, 1986), no exposed breccia pipes in the Grand Canyon 

region occur below the Redwall Limestone. Hundreds, however, occur above it. (2) Test 

drilling in one well-studied pipe the Orphan pipe confirmed that the breccia column 

terminated downward within the Redwall Limestone (Chenoweth, 1986). (3) The Redwall 

Limestone is honeycombed with numerous caverns, entrances to many of which are seen 

along the imposing cliffs of this unit throughout much of the length of the Grand 

Canyon. The bases of several pipelike bodies of breccia are exposed in some recently 

explored caves (Wenrich and Sutphin, in press). (4) Clasts within pipes have dropped to 

levels below their normal stratigraphic positions, indicating that all movement has been 

vertically downward.

DIMENSIONS

Vertical dimensions of individual pipes rarely are known with certainty, for the 

bottoms of few pipes are exposed and the tops of many have been removed by erosion. 

Nevertheless, exploratory drilling has established the minimum vertical extent of a few 

pipes. The well-known Orphan pipe, for example, cuts at least 1,660 ft (506 m) of section 

(Chenoweth, 1986), an amount that probably is not unusual. Diameters of most pipes 

range from about 100 to 400 ft (30 to 120 m), and 250 ft (about 75 m) may be taken as a 

practical average (Wenrich, Billingsley, and Van Gosen, 1986). Topographic expressions 

of pipes on plateau surfaces flanking the Grand Canyon, however, are as much as 5 times 

the actual pipe diameter (fig. 2) due to local removal, in solution, of gypsum and carbonate from soluble beds of the Toroweap Formation and Kaibab Limestone encircling the 

pipes (Wenrich, Billingsley, and Huntoon, 1986; Krewedl and Carisey, 1986). The resultant thinning of the affected strata and sag of overlying beds are expressed at the surface 

as structural basins considerably greater in diameter than the pipes themselves. The 

diameters of many pipe-related collapse depressions mapped by Sutphin (1986) on the 

southern Marble Plateau, for example, range from about 300 to 1,600 ft (90 to 490 m) and 

have an average diameter of about 850 ft (260 m).
BRECCIA CLASTS AND RING FRACTURES

Breccia clasts within pipes vary greatly in size, from small pebbles to large blocks 

and slabs more than 10 ft (3 m) on a side. The breccias generally have a sandy to silty 

matrix. Much of the matrix material probably represents the insoluble quartz component 

of calcareous sandstone clasts decemented by fluids circulating through the pipe, but 

downward sifting of loose sand from unconsolidated strata intersected by pipes has been 

cited as an alternative or supplemental source (Gornitz and Kerr, 1970; Chenoweth, 

1986). Movement of clasts within pipes has been exclusively downward, for all occur 

below their normal stratigraphic positions. Reported amounts of downdrop are 370 ft 

(113 m) at the Orphan pipe (Chenoweth, 1986), 500 ft (150 m) at the Ridenour pipe (this 

paper), and 740 ft (225 m) at the Kanab North pipe (Wenrich, 1986b). Displacements in 

some pipes increase from the margins of the pipe inward (Kofford, 1969; Gornitz and 

Kerr, 1970) and with increasing depth.

The contact between brecciated rock of the pipe core and the surrounding country 

rock is quite sharp in most pipes. Outside this contact, and extending for variable 

distances beyond it, the country rock commonly is cut by abundant, concentric, outwarddipping fractures that developed during stoping and that represent incipient foundering of 

large, arcuate slabs of rock around the pipe proper. This is the ring-fracture zone, a 

common site of rich concentrations of ore in mined pipes. Information on total widths of 

ring-fracture zones is scant in the literature; at the Ridenour mine it ranges from about 

40 to at least 130 ft (12-38 m).

MINERALIZATION AND ALTERATION

Mineralization and alteration of breccia pipes in the Grand Canyon region span as 

wide a stratigraphic interval as the pipes themselves, from the Mississippian Redwall 

Limestone to the Triassic Chinle Formation (Wenrich, 1985). U-Pb dating of uraninite 

ores from several pipes places the main stage of uranium mineralization at about 200-220 

Ma (Ludwig and others, 1986). The source and character of the mineralizing fluids have 

been long debated. Volcaniclastic sediments and tuffs of the Triassic Chinle Formation

have been cited as one convenient source for the uranium (Krewedl and Carisey, 1986), 

implying downward transport of U-laden waters in pipes or through connecting aquifers, 

but Hun toon (1986) states unequivocally that flow within pipes could only have been 

upward during mineralization. Wenrich (1986b) noted that the levels of enrichment of 

numerous metals within breccia-pipe ores are incompatible with derivation from the 

Chinle Formation and suggested instead that the uranium was derived from volcanic 

rocks of the Mogollon Highlands to the south, from where it was transported northward, 

down the regional hydrologic gradient, to the breccia pipes. Data on fluid inclusions 

from breccia-pipe ores suggest that temperatures during ore deposition ranged between 

80° and 173° C, and that salinities of the ore fluid generally were greater than 9 wt % 

eq. NaCl (Wenrich, 1985; Rasmussen and others, 1986; Wenrich and Sutphin, in press). 

These data would appear to preclude a low-temperature ground-water source for the ore 

and associated gangue minerals (Wenrich, 1986b).

A complex, multi-stage history of alteration and mineralization has been 

documented in several pipes for which detailed mineralogic studies have been completed 

or are in progress. Inasmuch as the timing of such events relative to fracturing of the 

host rocks is a principal focus of this paper, we briefly summarize the paragenetic 

sequence below.
(1) Bleaching and decementation of pipe breccia.

Although pipe-core breccias as seen in many places today are highly altered and 

locally mineralized, it is not difficult to imagine the basic character of a newly formed 

collapse breccia: a loose aggregate of poorly sorted, polylithologic, angular fragments 

resting in point contact with one another, and with abundant void space between. The 

exceptional permeability of such material would have continually exposed it to reaction 

with whatever fluids were circulating through the pipe at any given time. The earliest 

recognized effect, common to nearly all pipes studied in the Grand Canyon region, was 

removal of carbonate and consequent decementation of carbonate-cemented sandstone 

and siltstone clasts. Much of the stratigraphic section above the Redwall Limestone, 

including large parts of the Supai Group and Moenkopi Formation, consists of sandstone 

and subordinate siltstone cemented partly or wholly by calcite and dolomite. Freeing of 

individual quartz grains from clasts of such rocks was the probable source of most or all 

of the sandy matrix typical of pipe-core breccias. The same process resulted in rounding 

of those clasts most susceptible to solution. The resultant product, still preserved in the 

unmineralized portions of some pipes, was a polylithologic aggregate of breccia clasts 

some angular, some rounded in a porous matrix of loose quartz sand and sandy silt. The 

breccia in some places remains clast-supported, but matrix-supported breccias are 

common in other areas where dissolution of carbonate cement was more extreme.

Breccia within the pipes and strata immediately surrounding them commonly are 

bleached to ivory or buff tones, rendering some pipes, particularly those hosted by red, 

hematite-pigmented sandstones, visible from afar. The bleaching was effected by the 

reduction of finely divided hematite and redeposition of the iron in solution as disseminated pyrite (Gornitz and Kerr, 1970; Hoffman, 1977; Sutphin, 1986; Wenrich, 1986a? 

Gornitz and others, 1988). Considerable remobilization of iron is also indicated in many 

pipes by large masses of "zebra rock", an informal term used locally (albeit incorrectly) 

for bleached sandstones within which goethite-rich layers, generally noncoincident with 

bedding, alternate with goethite-poor layers to create a prominently striped appearance. Bleached and goethite-stained rock is of widespread occurrence in many pipes, 

whether mineralized or not. Conversely, none of the primary ore minerals discussed 

below are known to occur in unbleached rock. These observations led H.B. Sutphin (oral 

commun., 1987) and Gornitz and others (1988) to conclude that bleaching and remobilization of iron were early effects that predated ore mineralization within pipes.

(2)^Depositi6n o^carbonate and sulfate minerals.

Coarsely crystalline calcite, dolomite, and barite were among the most common 

and earliest of phases to be precipitated within the sandy matrix of the solution-collapse 

breccias of mineralized pipes (Wenrich and Sutphin, in press). Quartz was simultaneously 

removed, as shown by corrosion of once well-rounded detrital grains. Gypsum, anhydrite, 

siderite, and kaolinite are minor to rare phases assigned to the same stage of mineralization. The widespread deposition of carbonate minerals in rocks formerly depleted in 

carbonates apparently was common in the Grand Canyon breccia pipes. Later deposition 

of the ore minerals generally affected much lesser volumes of rock.

(3) Deposition of Ni-Co-Fe sulfides and Ni arsenides.

Deposition of small, scattered, euhedral and commonly zoned crystals of such 

phases as siegenite, bravoite, pyrite, and rammelsbergite corresponds to an intermediate 

stage of mineralization (Wenrich and Sutphin, in press). These minerals are hosted by the 

carbonate and sulfate minerals deposited previously. Massive, anhedral pyrite of slightly 

younger age locally enclosed the smaller crystals and crystal aggregates of the Ni-Co-Fe 

minerals by replacing the host grains of carbonate or barite.

10
(4) Deposition of uraninite and Cu-Fe-Zn-Pb sulfides.

The latest stage of mineralization discussed by Wenrich and Sutphin (in press) 

resulted in deposition of botryoidal aggregates of uraninite around detrital quartz grains 

and within small vugs in the calcite-cemented sand of the collapse breccia. Also important in some pipes, including the Ridenour pipe, was the deposition of such Cu and Cu-Fe 

sulfides and sulfosalts as chalcocite, digenite, chalcopyrite, bornite, djurleite, and 

covellite. Minor galena, sphalerite, and tennantite were also deposited locally at this 

time. The ores formed during this stage are compositionally and texturally complex and 

tend to occur within vertical, discontinuous, pod-like bodies within the pipe core and as 

vein arrays within the ring-fracture zone bordering the pipe proper (Gornitz and Kerr, 

1970; Krewedl and Carisey, 1986; Gornitz and others, 1988).

(5) Supergene alteration of primary ores.

Late Cenozoic uplift elevated the upper reaches of many pipes into the zone of 

oxidizing ground waters and resulted in extensive supergene alteration of primary ore. 

Common products include such Cu and Zn carbonates, sulfates, and hydrous silicates as 

malachite, azurite, brochantite, cyanotrichite, chrysocolla, hemimorphite, and smithsonite (Wenrich and Sutphin, in press). Goethite commonly is conspicuous and occurs 

variously as concretions, botryoidal masses, boxwork fracture fillings, and pseudomorphs 

after pyrite crystals (Wenrich, 1986b). Uranium in soluble complexes derived from the 

original uraninite was removed completely from the near-surface portions of some pipes 

and was locally redeposited in others as disseminated grains and fracture coatings of 

tyuyamunite and metatyuyamunite, uranophane, torbernite and metatorbernite, zeunerite 

and metazeunerite, uranospinite, and schroeckingerite (Chenoweth and Blakemore, 1961; 

Kofford, 1969; Sutphin, 1986; and data from this study). The Ridenour pipe is one of 

those in which supergene alteration of the primary mineral assemblage has been carried 

nearly to completion, at least within the visible portions of the pipe as reached through 

the mine workings.

FRACTURE NETWORK NEAR RIDENOUR MINE

The fracture network of the Paleozoic rocks near the Ridenour mine comprises two 

broad classes of fracture: (1) discrete faults, many of which are shown on geologic maps 

of the area, and (2) joints, which generally are not. Descriptions of the faults of the 

Grand Canyon region their origins, relative ages, and movement histories have 

appeared in more than a century of geologic literature. Summary papers by Huntoon 

(1974) and Lucchitta (1974) provide useful introductions to the topic. Faults in and near 

the Ridenour mine, however, are neither numerous nor large all are normal faults with 

less than 30 ft (9 m) of offset in nearly horizontal beds, and the largest fault on the mine 

property has an offset of only 3 ft (1 m). Local faults seem to have had little influence 

on the penetration of the Ridenour pipe through the Esplanade Sandstone, and our 

discussion of them is accordingly limited. On a regional basis, however, faults at deeper 

stratigraphic levels may have controlled the locations of some breccia pipes, and that 

topic is considered in a later section.

In contrast to faults, which are few near the mine, joints pervade every outcrop of 

the Esplanade Sandstone examined. Despite their ubiquity, published information on 

joints in Paleozoic rocks of the Hualapai Reservation was almost nonexistent until 

recently, when Roller (1987) worked in an area 35-50 mi (56-80 km) southwest of the 

mine site. Study of the fracture history of the Ridenour pipe and surrounding area is of 

more than academic interest, for some aspects of pipe genesis, mineralization, and 

supergene alteration cannot be fully understood without knowledge of the fracture 

network of the host rocks and the age of those fractures relative to pipe formation. The 

mechanics of upward stoping of an initial cavern by gravity collapse, for example, are

11
quite different for jointed versus unbroken rock. The movement of mineralizing fluids 

and later oxidizing ground waters through pipes also is strongly affected both by the 

structure of the pipe and of the surrounding fracture network in communication with it. 

The nature of that network changed with time and increased in complexity as successive 

sets of joints five in all developed in the rock. Additional complications stemmed from 

the gradual closure of old joints by precipitation of minerals within them, and by their 

local reopening during minor phases of crustal movement. Accordingly, the fracture 

history of the Esplanade Sandstone near the Ridenour mine is described below at some 

length.

METHODS OF STUDY

The local fracture network was documented in detail at 23 sites on the plateau 

surface capped by the Esplanade Sandstone. Four of these sites, stations RM-1 through 

RM-4, are within the ring-fracture zone along the periphery of the Ridenour pipe, and 

the other 19 are at various localities within an area of about two square miles (5 km z) 

surrounding the pipe (fig. 4). At these sites all visible characteristics of the joints 

(orientation, dimensions, shape, surface structures, mineral coatings or fillings, spacings, 

and the geometry of abutting or cross-cutting relations with other fractures) were 

recorded for the joints of each set. Many intervening areas were examined more cursoriiy.

Particular attention was paid to evidence of the mode of failure and relative age of 

each set, for from such information a partial stress history of the region can be reconstructed. Given reasonably favorable conditions of preservation, modes of failure (by 

shear or extension) among joints can be inferred from the detailed morphology of the 

fracture surfaces themselves. In the simplest case, if opposing walls of a fracture are 

irregular but perfectly matched, such that the absence of net shear offset can be demonstrated, the fracture most likely is an extension (Mode I) joint. If this condition is 

common to many joints of the same set, an extensile origin for the entire set can be 

regarded as established. An obvious limitation is that this method cannot be used to 

distinguish true shear fractures from extension joints later reactivated in shear. If, 

however, the fracture walls are relatively unweathered, at least locally, the presence of 

surface features such as plumose structure, twist hackle faces, and arrest lines can be 

taken as direct evidence of extensile failure. Detailed discussion of the formation and 

interpretation of these and other fracture-surface features is given in Kulander and 

others (1979), Pollard and others (1982), and Barton (1983).____________________

The relative ages of multiple fracture sets can be determined in the field primarily 

from abutting and cross-cutting relations among the various fractures. The criteria for 

shear fractures (faults) and filled dikes and veins are well established and generally 

known: younger faults offset older, and continuous, unbroken dikes or veins are younger 

than those they cut across. Relative-age criteria for unfilled extension fractures also 

exist (a good summary is given by Kulander and others, 1979) but are not yet in general 

use among field geologists. The prime consideration is that a tensile stress cannot cross 

a cohesionless boundary. Thus, a given extension fracture will terminate against any 

older fracture unless the opposing walls of the older fracture are bonded together in 

stress-transmitting contact, generally through cementation by some secondary mineral 

(Grout and Verbeek, 1983). Even if the older fracture is "healed", however, the mechanical contrast between the wall rock and the mineral filling often is sufficient to stop a 

younger joint from propagating across the contact. Depending on the degree of cohesion 

between fracture walls, then, younger extension joints will either terminate against, or 

cut across, older fractures.

A more complete discussion of field methods used in this study and their application to various topics concerning joints in general is given in Grout and Verbeek (1983).

12
Figure 4. Generalized topographic map of area around Ridenour mine, showing location 

of fracture stations in Esplanade Sandstone. Stations RM-1 to RM-4 are within the 

ring-fracture zone of the Ridenour pipe. Hachured line is rim of the Inner Gorge of 

the Grand Canyon. Map from Vulcans Point S\V quadrangle, 1:24,000 scale.

13
DESCRIPTION OF JOINT SETS

A first impression of the fracture network of the Esplanade Sandstone near the 

Ridenour mine can be gained from aerial photographs, for the openings of many of the 

larger joints have been accentuated by weathering and are prominently visible from 

afar. The map of Figure 5, prepared from true-color aerial photographs at 1:27,000 

scale, discloses several fracture sets of consistent orientation but varying relative 

prominence from place to place. Because the fractures are nearly vertical and the 

mapped surface is a plateau with only gentle relief, fracture trends mapped from vertical 

aerial photographs are equivalent to fracture strikes as measured on the ground. A set of 

about N60W strike appears dominant in most areas, but fractures of other trends 

particularly about N-S and N70E are common locally. Only the most prominent 

fractures are visible on the photographs, however, and the actual fracture network as 

documented in the field is considerably more complex than that suggested by Figure 5. 

The results of the field studies are consistent from site to site and show that the local 

joint network of the Esplanade Sandstone is the product of five sequential episodes of 

fracture. Joints formed during those episodes are referred to in the descriptions below 

as Jl (oldest) through J5 (youngest).

Episode 1: N-S joints

Prominent, nearly vertical Jl joints of N8W-N4E strike are present at 7 of the 23 

sites studied (fig. 6). At each site they were the earliest set to form. These joints are 

extension fractures, as shown by prominent plumose structure and local twist hackle on 

some of their faces where these have been most protected from weathering. Their 

surfaces are planar, or nearly so, and are of large size exposed lengths of 15-30 ft (5-10 

m) and exposed heights of 10-15 ft (3-5 m) are common in many areas. The full dimensions of these (and other) joints are best viewed along the canyon rim, where apparently 

single Jl surfaces, as much as 50 ft (15 m) high and 140 ft (43 m) long, form broad, flat 

cliff faces (fig. 7).

The Jl joints are sporadically distributed and are absent from much of the study 

area. In many places only one or two large Jl joints are present within outcrops hundreds 

of feet across. Locally, however, they are present in sufficient numbers to constitute a 

recognizable set, and in a few outcrops they are spaced only 1-3 ft (0.3-0.9 m) apart. 

Domains of abundant Jl joints tend to be small, in some places only 20-40 ft (6-12 m) 

wide, but soil cover on the plateau surface near the mine is too extensive to permit 

determination of the shapes of those domains or to recognize any pattern among them.

Episode 2: N70E joints

Subvertical J2 joints, ranging in strike from about N60E to N79E, are present in 

many localities and were documented at 16 of the 23 stations examined in detail (fig. 

8). In appearance these joints much resemble those of the Jl set: they tend to be large, 

with exposed heights of 20-50 ft (6-15 m) and trace lengths of 25-70 ft (8-21 m), and 

their surfaces are planar to gently sinuous. The traces of some apparently single J2 

joints on aerial photographs are 300-450 ft (90-140 m) long. Such large J2 joints cut 

multiple beds of sandstone and form vertical cliff faces at many points along the canyon 

rim north of the mine. Smaller J2 joints with trace lengths of only 1.5-6.0 ft (0.5-2 m) 

are present within thin sandstone beds at a few localities, but these are uncommon.

The prominence of the J2 joint set is highly variable from place to place, as 

suggested by photogeologic evidence (fig. 5) and confirmed in the field. At a few 

stations the J2 joints are spaced as little as 3 inches (8 cm) apart and divide the rock into 

thin vertical slabs. More commonly, spacings range from 3-10 ft (1-3 m), but in some 

areas only one or two J2 joints are seen over large expanses of outcrop, and at still other

14
Figure 5. Photogeologic fracture-trace map of Ridenour mine area, prepared from 

1:27,000 true-color aerial photographs. Shown at same scale as mapped. RM = 

Ridenour mine.

15
Figure 6. Orientations of Jl joints in Esplanade Sandstone around Ridenour mine. Most 

symbols represent the median of 12-20 readings; dashed symbol indicates sparse 

data. Overall median for entire area is NIW.

16
Figure 7. Large cliff-forming Jl joints along canyon rim west of Ridenour mine.

locations they appear to be missing entirely. As with the Jl set, no obvious pattern to 

their distribution as yet has been recognized.

Coexistence of J2 joints with the sporadically distributed Jl joints is sufficiently 

uncommon at the outcrop scale that determination of their relative ages was difficult. 

The problem was compounded by the fact that many joint-bounded blocks near the 

canyon rim, where exposures are best and soil cover least, are slightly out-of-place due 

to movement of the blocks near the unsupported cliff face. Abutting relations of one 

joint against another in some such areas are suspect. Nevertheless, apparently clear 

terminations of J2 joints against Jl surfaces were seen in several places, suggesting that 

J2 is the younger set. Plumose structure observed on numerous J2 faces confirms the 

extensional origin of the set, but J2 surfaces in many of the weakly cemented sandstone 

beds are far too weathered to preserve such delicate features.

17
Esplanade 

Platform

87

Figure 8. Orientations of J2 joints in Esplanade Sandstone around Ridenour mine. Most 

symbols represent the median of 12-20 readings; dashed symbols indicate sparse 

data. Overall median for entire area is N68E,
Episode 3: N60W joints

The J3 joints, of N49W-N63W strike and nearly vertical dip, comprise by far the 

most prominent joint set of the Esplanade Sandstone near the Ridenour mine (fig. 9). 

Few outcrops lack a definable J3 set (they are present at 21 of the 23 stations studied), 

and in many of these the J3 joints dominate the local fracture pattern. Joints of this set, 

as mentioned previously, also are prominent on aerial photographs of the region. 

Distances between adjacent 13 joints locally are as little as 3 inches (8 cm) and elsewhere as great as 15 ft (5 m), but spacings of 1.5-6.0 ft (0.5-2 m) are most common. The 

13 joints generally are planar where they comprise the only well-expressed set, but their 

surfaces are noticeably more irregular wherever older joints are abundant. The typical 

appearance of the 13 set is shown in Figure 10.

A notable aspect of 13 joints is their relatively small size, much smaller on average 

than the joints of the preceding two sets. 13 joints at most stations are only 2-6 ft (0.6-2 

m) long, and at only two stations were lengths of 10-20 ft (3-6 m) common. In addition, 

the 13 joints, more so than either the Jl or J2 joints, tend to terminate against 

prominent bedding surfaces between adjacent ledges of sandstone. Many J3 joints are 

thus confined to single beds and have heights equal to the thickness of those beds, 

typically 3-12 ft (about 1-4 m). Other but less common J3 joints cut multiple beds and 

have heights fully equal to the largest Jl and J2 joints, from 20 to 50 ft (6 to 15 m). 

Some of these are single fractures, but others, on close inspection, are seen to be 

vertically extensive zones of smaller, overlapping J3 joints rather than single fracture 

surfaces. The tendency for J3 joints to congregate in zones, within which individual 

fractures are spaced only fractions of an inch apart, is a property most pronounced in 

this set.

Abundant plumose structure, twist hackle, and local arrest lines show that the J3 

joints are extension fractures. The configuration of plumose structure on some J3 joints 

shows that they propagated outward from a point on a preexisting Jl or J2 surface and 

establishes J3 as the younger set. So too does the fact that many other J3 joints curve 

toward and terminate against members of the Jl and J2 sets at all stations where these 

sets coexist. At only one station were large numbers of J3 joints observed to cut across 

the earlier fractures, implying local cementation of those earlier fractures prior to 

formation of the J3 set.

Episode *: Horizontal, bed-parallel joints

Fractures more-or-less parallel to bedding, and analogous to sheeting (exfoliation) 

joints formed by erosional unloading of crystalline rocks, are common in the Esplanade 

Sandstone. Many such fractures, here termed J4 joints, occur along the bedding-plane 

interface between beds of slightly different lithology. Others occur within single beds 

and are of variable form, depending in large part on lithology. Where the rock is planeparallel laminated, as is common in the siltstones but less so in the sandstones, the joints 

too are planar and follow the depositional surfaces (fig. 11, bed A). Such joints in the 

finer-grained beds are in part responsible for the fissile appearance of those beds. Within 

thick, massive, visually structureless sandstones, however, the J4 joints are irregular, 

curving fractures only crudely parallel to the top and bottom surfaces of the bed (fig. 11, 

bed C). Still other J4 joints in crossbedded, weakly to moderately cemented sandstones 

formed along the interface between adjacent cross strata and thus are inclined at various 

low angles to the other types (fig. 11, bed B). And finally, J4 joints in some wellcemented beds of crossbedded sandstone cut through the cross beds and maintain a 

subhorizontal attitude (fig. 11, bed D). All four types are widespread and gradational to 

one another.

In many exposures of the Esplanade Sandstone the J4 joints terminate against the 

walls of J3 joints. The J4 joints thus are relatively young members of the total fracture 

system.__________ ____________ ______________________________

19
Figure 9. Orientations of J3 joints in Esplanade Sandstone around Ridenour mine. Most 

symbols represent the median of 12-20 readings. Overall median for entire area is 

N59W. _ __

20
Figure 10. Photograph of well-developed J3 joints (parallel to line of sight of observer) 

in Esplanade Sandstone near station RM-9. Small, flat joints facing observer are 

members of J5 set.

 ."''' '''.''jj.i''' v.'.'.'.' '-'.'*' f *'.'V'-'' V'*j'' V'" '**""

 " .^^ ^^^^^ ivMM^tfMrfn^WMH^^^HI^^V^tt

B

D

Figure 11. Sketch showing influence of lithology on character of J4 parting joints in the 

Esplanade Sandstone. __________

21
Episode 5: N35E joints

Small, crudely formed 15 joints of N25E-N50E strike and subvertical dip form a 

widespread but generally weakly expressed set near the mine (fig. 12). These joints are 

everywhere nearly perpendicular to the earlier 33 set (horizontal angles between them, 

as measured clockwise from 33 to 35, range from 82° to 101°) and have a similar 

distribution: they are present at most (14 of 21) sites where the 33 set was documented, 

but are missing from all studied outcrops that lack the 33 set. Despite the seeming 

connection, however, the two sets are of much different age, appearance, and genesis. 

Nearly all 35 joints terminate against whatever 33 (or older) joints are present and thus 

have lengths equal to or less than the spacings between those joints. Those lengths, 1-5 

ft (0.3-1.5 m) in most places, are far shorter than typical lengths of 31, 32, and many 33 

fractures. The heights, too, of 35 joints are small because few cut the entire thickness 

of any given sandstone bed; instead they are confined to individual slabs of rock bounded 

top and bottom by adjacent parting joints of the 34 set. Most 35 joints range in height 

from 1 to 4 ft (0.3 to 1.2 m), again significantly less than typical heights for the joints of 

all older sets. The small size and subplanar to nonplanar surfaces of 35 joints are 

properties shared consistently by no other set.

Although widespread, only rarely are 35 joints so closely spaced as to constitute a

major element of the local fracture network. Spacings from 1-3 ft (0.3-1 m) were 

documented at only a single locality; in most other places the 35 joints are spaced 5-25 ft 

(1.5-7.5 m) apart. Arrest lines, twist hackle, and plumose structure are commonly found 

on 35 joint walls and show that they, like the members of the other sets, originated as 

extension joints.

COMPARISON OF FIELD AND PHOTOGEOLOGIC STUDIES

The traces of thousands of individual fractures are visible on true-color aerial 

photographs (about 1:27,000 scale) of the Esplanade Platform near the Ridenour mine 

(fig. 5). Nearly all are joints, although a few of the longest traces on the map represent 

minor faults. Most of the joints are visible directly as solution-widened clefts narrow 

lines of shadow across a sunlit rock surface and thus present no difficulty to a photointerpreter interested in fracture networks. Others, hidden beneath a thin veneer of silty 

soil, are disclosed by rectilinear alignments of closely spaced desert shrubs. Although 

such alignments cannot unequivocally be proved to represent fractures, their similar 

orientations and lengths to nearby joints in bare rock leave little doubt as to their 

identity. Their numbers, in any event, are few compared to joints visible directly.

The fracture-trace map of Figure 5 is compounded almost entirely of three major 

fracture trends that correspond to the 31, 32, and 33 joint sets. The following characteristics of these joints as seen on the photographs were confirmed during field studies.

(1)31 joints, of approximately due N strike, are large, only modestly abundant, and 

are irregularly distributed across the Esplanade surface.

(2) 32 joints, of approximate N70E strike, are also large, modestly abundant, and 

irregularly distributed.

(3) 33 joints, of approximate N60W strike, comprise by far the most abundantly 

visible joints and the most uniformly widespread set. Far more 33 joints were seen on 

the photographs than could be mapped at the scale of Figure 5. Their traces are notably 

shorter, on average, than those of either of the previous two sets, in keeping with their 

smaller size as observed in the field. Many 33 traces as seen on the photographs probably 

represent narrow zones of aligned 33 joints rather than individual fractures, most of 

which are significantly shorter than the traces on Figure 5 might imply. Thousands of 33 

joints are visible in stream valleys where much bare rock is exposed, but their traces are 

less abundant on the adjacent plateau surfaces where thin veneers of unconsolidated 

sediment tend to obscure most joints.__________________________________

22
Figure 12. Orientations of 15 joints in Esplanade Sandstone around Ridenour mine. Most 

symbols represent the median of 12-20 readings. Overall median for entire area is

N33E.

23
Overall, the aerial photographs provide an accurate view of the orientation and 

relative prominence of three major joint sets in the Esplanade Sandstone, and provide 

some evidence of their distribution as well. Moreover, had one endeavored to guess the 

relative age of the Jl through J3 sets from photographic evidence alone, the guess 

probably would have been correct. Such deductions rest on the assumption that the 

oldest joints are the longest because no older fractures existed to impede their lateral 

growth and that fractures of successively younger sets are progressively shorter. The 

average trace length of the joints of any particular set can then be taken as a crude 

guide to its relative age. The traces of Jl joints tend to be of uniformly substantial 

length wherever they are found and thus, by that criterion, the Jl joints probably 

comprise an early set. Some J2 joints, too, are of impressive length, but others are 

shorter, particularly in areas of abundant Jl. In such areas it is rare to observe a J2 

trace cutting across a Jl joint, and so the J2 joints appear to be the younger set. Finally, 

the myriad short traces of the J3 joints and the rarity with which they appear to cross 

other fractures suggest they are the youngest of the three prominent sets visible on the 

photographs. The relative ages of the three sets as documented on the ground are in 

good accord with properties of the individual joints as seen from the air.

Although it would certainly appear that photogeologic work in the Ridenour mine 

area could have provided a useful shortcut to structural studies, it must be emphasized 

that fracture histories as read from aerial photographs alone should neither be assumed 

to be true nor claimed to be complete. Demonstration that one joint set predates 

another necessarily rests on direct observation of abutting relations in outcrop, and 

sometimes with a hand lens. The distant view provided by aerial photographs should 

never be assumed to provide reliable information on such small-scale effects. Then, too, 

bed-parallel joints such as those of the J4 set are not visible on vertical photographs of 

horizontal beds, and so the photographs contain no evidence of at least one critical 

element of the fracture and uplift history of the area. Finally, the photographs used for 

this study were insufficient to reveal the widespread distribution or even the certain 

existence of the J5 joint set, due to the small size of those fractures. Our experience in 

other parts of the Grand Canyon region leads us to a similar conclusion: aerial photographs are a valuable tool to be used in structure studies, but considerable field work is 

mandatory to any believable interpretation of fracture history.

TIMES AND DEPTHS OF JOINTING

34 and 35 sets

As in many other areas of horizontally bedded sedimentary rock, the presence of 

bed-parallel parting joints in this case, the J4 set provides a key for distinguishing old, 

pre-uplift joints from those formed later, during or after regional uplift and unloading. 

The pre-uplift joints presumably formed at deeper crustal levels, higher confining 

pressures, higher temperatures, and perhaps under conditions of higher fluid pressures 

than did their younger counterparts.

Bed-parallel parting joints in sedimentary rocks are analogous to sheeting joints in 

crystalline rocks but have been comparatively little studied. Recent work has shown that 

they can form within the upper 1,600 ft (500 m) or so of section (Hickman and others,

1985) and that, with local exceptions due to favorable lithology, their numbers decrease 

with depth. In one underground mine in Colorado, for example, bed-parallel joints in oil 

shales at the 490-ft (150-m) level are four times as abundant as their counterparts at the 

bottom of the mine, 340 ft (100 m) lower (E.R. Verbeek and M.A. Grout, unpub. data,

1986). Such studies suggest that, as in crystalline rocks, the presence of abundant 

subhorizontal extension fractures in little-deformed sedimentary rocks is indicative of 

fracture under relatively near-surface conditions. If such conditions were approached

24
only once during the burial and uplift history of a given rock unit, such that the maximum 

age of the bed-parallel partings within that unit can be stated with some degree of 

certainty, then the maximum age of some joint sets and the minimum age of others can 

also be estimated.

Wenrich, Billingsley, and Huntoon (1986) suggest that regional uplift of the 

Hualapai and western Coconino Plateaus began during the compressive movements of the 

Laramide Orogeny, which they define in a broad sense to include Late Cretaceous 

through Eocene events. Uplift continued at intervals through the whole of Cenozoic 

time, ultimately elevating the region by 2-3 miles (3.2-4.8 km) and resulting in the 

stripping of thousands of feet of rock from the plateau surfaces since the close of the 

Cretaceous Period some 65 Ma (Wenrich, Billingsley, and Huntoon, 1986). Exactly when 

during this extended cycle of uplift the J4 joints formed in the Esplanade Sandstone is 

conjectural. Two facts, however, suggest that they are geologically quite young. First, 

knowledge that unloading joints form only at shallow depths implies that most of the 

overlying rock had already been stripped from the Esplanade Sandstone by the time the 

J4 joints formed. Second, Lucchitta (1979) presented evidence that more than 3,000 ft 

(900 m) of uplift along the Grand Wash cliffs, 40 mi (64 km) west of the Ridenour mine, 

has occurred only in the last five million years. That 20-30% of the total uplift occurred 

during the latest 7-8% of Cenozoic time suggested to Wenrich, Billingsley, and Huntoon 

(1986) that rates of uplift (and thus erosion) have accelerated in the recent geologic 

past. These two facts, taken together, suggest a late Cenozoic age for the J4 and J5 

joint sets.

Additional evidence refines the probable age of the 34 and J5 sets still further. 

The relatively uniform orientations of the J5 joints across the Esplanade surface (fig. 12) 

imply the former existence, during J5 time, of uniformly oriented stresses within that 

unit, and hence the absence of steep local topographic gradients. Put another way, had 

erosional dissection of the Esplanade Sandstone already begun by J5 time, the J5 joints 

near the sites of incision that is, near the present-day edges of the Esplanade Platform- 

-could not have formed in the orientations they did. We thus conclude that the J5 joints 

formed before the Grand Canyon reached less than about half of its present depth. 

Canyon cutting at higher levels must nevertheless have already been in progress, for the 

presence of the J4 unloading joints shows that most of the Permian and Triassic strata, 

still preserved on nearby plateaus flanking the Grand Canyon, had been removed from the 

Ridenour mine area. Both the J4 and J5 sets, then, must date from the early stages of 

cutting of the western Grand Canyon. Lucchitta (in press) recently summarized evidence 

that this section of the canyon was excavated "in, at most, the interval between 6 Ma . . 

. and 1 Ma. More likely, it began to be cut shortly before 5 Ma and was nearly as deep as 

it is today shortly after 4 Ma." If so, the most probable age of the J4 and slightly 

younger J5 joints is around 5 Ma, or early Pliocene.

A postulated early Pliocene age for the J5 joint set is consistent with evidence that 

crustal stresses at that time were favorably oriented for the formation of a set of NEstriking joints. Crustal extension, beginning in Miocene time and continuing into the 

Pliocene, resulted in extensive normal faulting of the Grand Canyon region (Lucchitta, 

1974; Huntoon, 1974), first along N-trending faults and somewhat later along those of NE 

trend. These movements were related by Huntoon (1974) to the onset of the Basin and 

Range Orogeny in areas to the west.

Jl through J3 sets

Nearly 270 million years elapsed between deposition of the Esplanade Sandstone in 

early Permian (Wolfcampian) time (McKee, 1982) and formation of the J4 and J5 sets in 

Paleocene time. Three sets of joints formed in the sandstones during this long interval, 

but determination of their absolute ages is difficult at present. Evidence to be presented 

in a later section, however, will show that all three sets are of post-late Triassic age, and

25
thus younger perhaps much younger than 200 Ma. The general absence from the region 

of strata younger than Triassic frustrates attempts to date the fracture events more 

closely through conventional means. The J2 set in the Esplanade Sandstone possibly is 

Laramide in age because its joints strike about N70E, parallel to the direction of crustal 

compression during monocline development (Reches, 1978), but this is a tenuous suggestion at best.

Inasmuch as the Jl through J3 sets almost certainly predate canyon cutting, the 

vertical distance from the Ridenour mine to the top of the Kaibab Limestone, which caps 

the nearby plateaus, provides a minimum estimate of overburden loads at the times of 

jointing. That distance is 1,900-2,000 ft (560-590 m). If one or more of the J1-J3 sets 

formed before regional denudation had stripped the area to the Kaibab surface that is, 

if some of the sets are pre-Laramide then the thickness of the overlying Moenkopi 

Formation must be added. About 1,200 ft (360 m) of Moenkopi is preserved at Mt. 

Trumbull 21 mi (34 km) to the north and also probably covered the Ridenour mine area 

(G.H. Billingsley, oral commun., 1988). The minimum thickness of rock that once overlay 

the Ridenour strata, then, is about 3,100-3,200 ft (945-975 m). Whether any younger 

sediments of the Chinle and overlying formations were once deposited in this area is 

conjectural. Regardless of the uncertainties involved, it is clear from these simple 

arguments that the J1-J3 sets represent fracturing of the rock under conditions quite 

unlike those of the shallowly formed, and younger, J4 and J5 sets.

A COMMENT ABOUT JOINT STYLES

The relation between environmental conditions during fracture and the physical 

characteristics of the fractures produced is a complex one, understood only in general 

terms for most geologic applications. Several properties of the joint network near the 

Ridenour mine nevertheless seem worth noting, inasmuch as they clearly reflect changing 

conditions of fracture through time.

Fractures of the early Jl and 12 sets, which formed in moderately to deeply buried 

strata below the level at which unloading joints can form, commonly are of large vertical 

size. Many grew to heights of 5 m or more and cut multiple beds of sandstone, terminating vertically only against intervening beds of mudstone. Joints of these two sets are of 

such closely similar size and appearance that conditions during fracture probably were 

much alike. A change in conditions was signalled by the appearance of the J3 set, whose 

joints show a greater tendency to terminate against bedding surfaces than do their 

predecessors. This suggests as one possibility that fracture was now occurring at a 

shallower depth. Finally, the most shallowly formed joints, those of the J5 set, are 

almost uniformly of small vertical extent because their growth in that direction was 

restricted by the then-common subhorizontal joints of the J4 set. Decreasing joint age 

and decreasing depth of fracture, then, were accompanied in this area by a general 

decrease in joint height.

Studies by two of us (Verbeek and Grout, 1983, 1984; Grout and Verbeek, 1985) in 

several other areas of the western U.S. have disclosed broadly similar results. In areas 

where the geologic history is not clouded by multiple episodes of uplift and reburial, the 

earliest (and thus most deeply formed) joints commonly terminate only against contacts 

between strata of dissimilar lithology. Where the beds are thick or where multiple beds 

of similar lithology are in contact, these joints are of large vertical extent. Joints of 

later sets known or inferred to have formed at somewhat shallower depth often show a 

tendency to terminate against more minor lithologic contacts, and are thus smaller. 

Post-uplift joints that formed in near-surface rocks already pervaded by unloading joints 

generally are the smallest of all. It is often possible, then, to distinguish at a glance 

which among several sets of coexisting fractures formed early and at an appreciable 

depth, and which formed later and nearer the surface. We stress, however, that this is a

26
general tendency only, with known exceptions, and that joint size should not by itself be 

considered a reliable criterion of either relative age or depth of formation.

MINERALIZATION AND ALTERATION ALONG JOINTS

Jl through J3 joints

Alteration of the sandstone wall rock adjacent to 31 through 33 joints was noted at 

many localities. The most common effect is bleaching of the normally pink or brick-red 

sandstone to pale shades of orange-gray or tan, rendering many joints highly visible as 

prominent stripes across pavement outcrops. The degree to which the walls of individual 

joints are altered shows obvious correlation with joint age: where joints of two different 

sets coexist in the same bed, bleached zones along joints of the older set are invariably 

wider than those along joints of the younger set. Near station RM-22, for example, wall 

rock adjacent to 32 joints is bleached for distances of 0.2-0.3 inch (5-8 mm) from the 

joint planes, whereas for 33 joints the corresponding distances are only 0.08-0.16 inch (2- 

4 mm) (fig. 13). In another outcrop nearby the bleached zones extend 0.04-0.12 inch (1-3 

mm) to either side of 33 joints, but 35 surfaces are not bleached at all. Observation that 

the oldest joints are consistently the most altered suggests that the joints of each set, 

once formed, generally remained open to ground-water flow thereafter rather than being 

quickly sealed by precipitation of secondary minerals within them.

The comparative rarity in the area of joint intersections, as opposed to simple 

terminations, shows that complete closure of the joints of any one set by cementation 

prior to formation of the subsequent set was only locally achieved. This, too, suggests 

that the joints of any given set generally remained open to ground-water flow long after 

their formation. However, partial closure by precipitation of secondary minerals as thin 

films on joints was rather common. Careful examination with a hand lens of horizontal 

pavement exposures reveals in many places the presence of thin coatings of colorless to 

white, coarsely crystalline calcite on the walls of 31 through 33 joints. In most joints the 

calcite grew upon walls already discolored (bleached) by ground-water flow, but in some 

the calcite was deposited upon a pristine joint wall, implying precipitation shortly after 

joint opening. In a few areas a hair-thin film of a nearly black mineral (goethite?) 

intervenes between the joint wall and the white calcite. The wall rock itself commonly 

is impregnated with calcite for distances of 0.1-0.4 inch (3-10 mm) outward from the 

joint planes and thus is both more firmly cemented and less porous than the adjacent, 

unaffected sandstone more distant from the joints. Even where prolonged weathering has 

stripped all trace of mineral coatings from the 31 through 33 joint walls, as it has in 

much of the area, the tendency for the adjacent well-cemented rock to protrude as thin 

ribs from weathered outcrop surfaces attests to carbonate mineralization along the 

fractures at some time in the past.

J4 and J5 joints

Opportunity to document the effects of alteration and mineralization along 34 

joints was limited because the sandstones, upon exposure, flake readily parallel to 

bedding. Nevertheless, probable remnants of thin calcite coatings, now highly 

weathered, were noted in a few places. Thin infillings of translucent white calcite were 

likewise noted within 35 joints in several places where these joints were best protected 

from weathering. Bleaching of the wall rock adjacent to 34 or 35 joints, however, was 

not observed at any locality.

27
Figure 13. Bleached (reduced) rock adjacent to 12 and 33 joints near station RM-22. 

Photograph of horizontal surface. Note that widths of bleached zones are wider for 

the older 32 joints (parallel to long edge of photograph).

Discussion

The overall picture that emerges from the evidence available is one of prolonged 

ground-water flow through a well-interconnected network of 31 through 33 joints, with 

only incidental (and geologically late) contribution to the flow network by the younger 

and generally tighter 34-35 joints. Detailed study of the mineralogic and chemical 

effects accompanying alteration of joint walls and deposition of secondary minerals upon 

them is beyond the scope of this study, but a few general conclusions are evident from 

field observations:

(1) Fluids circulating through 31-33 joints were capable of removing finely divided 

hematite pigment from the original rock to leave narrow bleached zones adjacent to the 

joint walls. The reducing agent presumed responsible for this process remains unidentified. The absence of visible pyrite and rarity of goethite within the bleached rock or 

coating the joints suggests that most of the iron was removed in solution rather than 

being redeposited locally in some other form. (The limonite commonly seen on exposed 

joint walls along the canyon rim is a much younger product of weathering.)

(2) The absence along 34 and 35 joints of bleached zones comparable to those along 31-33 

joints shows that the reducing fluids migrated through the Esplanade Sandstone before 

uplift, while that unit was still deeply buried. Upon subsequent uplift and erosional 

unloading, when the 34 and 35 joints formed in the shallow subsurface, the Esplanade 

Sandstone was subject only to the influences of shallowly circulating, oxidizing, meteoric

28
ground waters. The joint network shows clearly the effects of changing fluid chemistry 

with time.

(3) None of the fluids circulating through 31-J5 joints were at any time capable of 

removing carbonate from the host rocks to widen the joints by solution. As shown later, 

this stands in sharp contrast to the extreme effects of solution-widening along ring 

fractures within the same rocks adjacent to the Ridenour pipe. Evidence for the lack of 

solution along joints is seen throughout the area and takes several forms: (a) The 

opposing walls of many joints are irregular in detail but almost perfectly matched, such 

that the fracture could be closed completely if pressed together. Dissolution of wall 

rock adjacent to joint surfaces would destroy this relationship, (b) The preservation on 

many joints of delicate surface features that formed during fracture propagation shows 

that the joint faces, though altered in many examples, have retained their original 

form. Plumose structure on most joints, for example, cannot survive removal of material 

more than one or two grain diameters inward from the original joint walls, yet joint 

plumes are magnificently preserved on many joints and have been modified only by 

weathering upon exposure, (c) The calcite fill in all joints is clean and lacks inclusions of 

quartz grains decemented from the adjacent wall rock.

(4) Calcite was deposited from both reducing and oxidizing solutions as a stable phase 

during nearly the entire fracture history of the Esplanade Sandstone. Evidence for this 

stems from the manner in which the joints of various sets abut and crosscut one 

another. Although fracture intersections are relatively uncommon, implying a fairly 

open fracture network as noted previously, their local presence is instructive as to timing 

of mineralization within joints. Intersections of the following types have been 

recorded: J2 joints across Jl, J3 across Jl and J2, and J^ and J5 across joints of all 

previous sets. The only common phase within all these fractures is calcite. These 

observations imply that deposition of calcite within the joints of any one set had already 

begun by the time the succeeding set formed, thereby enabling those joints already 

bonded together in stress-transmitting contact to be crosscut by members of the younger 

set. The picture that emerges is one of slow deposition of calcite over a protracted 

period of time, during which multiple fracture sets were created in the Esplanade 

Sandstone. Rates of deposition were such that most joints were slowly plated with 

calcite, but not sealed completely, thus remaining open to fluid flow. In a few places, 

however and particularly among small joints of narrow original aperture mineralization 

was complete and younger fractures cut across older, sealed joints.

Post-exposure effects

A very late stage of mineralization is recorded by the presence of a highly calcareous, fine-grained, porous material that filled all remaining fracture space in many 

outcrops in the study area. This material is variously white, pale gray, or tan, has a low 

specific gravity due to its great porosity, and has a chalky appearance resembling 

travertine or caliche. Observations in the Ridenour mine confirm the latter origin, for 

the chalky fracture fillings exist only within the shallowest mine workings and fail to 

persist to depth. Fillings of caliche within Jl through J3 joints commonly are 0.1 to 0.8 

inch (0.3-2.0 cm) thick but in some places attain thicknesses of 1-2 inches (2.5-5 cm); 

analogous fillings in 3b and J5 joints, in contrast, rarely exceed 0.3 inch (0.7 cm). Grains 

of fine quartz sand, weathered from the Esplanade Sandstone and blown or washed into 

open fractures, are embedded in caliche at many sites. Locally these are accompanied 

by angular fragments of wall rock to form a sandy, calcareous, matrix-supported 

"breccia" bearing a superficial resemblance to concrete. The young age of the caliche is 

reflected by its presence in joints of every set in the area as well as by its form as 

continuous, unbroken fillings extending from one weathered fracture into others of 

different orientation (fig. 14).

29
r ^E«^ C'"

Figure 14. Continuous filling of caliche within open, weathered 33 and 35 joints. 

Photograph of horizontal pavement surface. 33 joints are parallel to tape.

REACTIVATION OF JOINTS

The joints of all five sets of the Esplanade Sandstone around the Ridenour mine 

formed as extension fractures, but evidence of reactivation of several sets was seen at 

many localities. Although detailed study of the directions and sequence of such movements could add much new information to the structural history of the region, especially 

as regards stress directions and movements on nearby faults, such study was beyond the 

scope of our work. Below we record only a few observations, together with brief 

comments on their possible tectonic significance.

The realization that some joints were sheared laterally subsequent to being opened, 

whereas others of different orientation were simply opened further without shear, has 

proved intriguing. Although many joints in the area are too highly weathered to permit 

easy recognition and measurement of small amounts of lateral slip, such slip did occur 

and probably was common. Between stations RM-5 and RM-8, for example, the evidence 

for horizontal shear along some joints was especially clear. In this area, within a 

complex zone of interaction between 32 and 33 joints, oblique right-lateral reopening of 

numerous 32 joints was documented by matching points along one irregular joint wall to 

their offset counterparts on the opposing wall. The measured strains ranged from 0.08 to 

0.3 inch (2-8 mm) in a N20E-N^5E direction, roughly perpendicular to the 33 joints. The 

movements thus probably date from the time of formation of the 33 set.

Continuation of the same strain whose initial increments gave rise to the 33 joint 

set could well explain a variety of observed structures in the Ridenour mine area. The 

following events, from oldest to youngest, are possibly all related to a single, protracted 

period of NNE- to NE-directed, horizontal crustal extension: (1) Formation of abundant 

33 joints and concomitant shear along preexisting 32 joints. (2) Widening of some 33 

joints to form extensional fissures as strain continued. (3) Local inception of vertical 

movement along 33 surfaces as the strata were gradually pulled apart. At station RM-2,

30
for example, stratigraphic separations of 1-4 inches (about 3-10 cm) occurred along 33 

joints and resulted in segmentation and offset of previously mineralized ring fractures 

near the Ridenour pipe (fig. 15). (4) Map-scale faulting as offset along 33 joints 

increased with continuing extension. The series of WNW-striking faults that offset the 

Esplanade Sandstone northeast of the Ridenour mine (Wenrich, Billingsley, and Huntoon, 

1986) may be structures of this type they are mapped as having vertical rather than 

inclined dips and likely represent reactivation within a linear zone of previously formed 

33 joints. Though such structures are common on the Colorado Plateau and invariably 

are portrayed as faults on geologic maps (see McGill and Stromquist, 1979, for spectacular photographs), it is important to realize that they did not originate through shear 

failure of the affected strata.

The above scenario implies that some normal faulting in the Ridenour mine area 

and particularly along WNW- to NW-trending faults parallel to the 33 set occurred prior 

to formation of the bed-parallel 34 set, and thus before the main phase of regional 

uplift. If so, movement on those faults predated movements along the much more 

numerous faults that strike NNW to NNE and that were either created or reactivated in 

late Cenozoic time during E-W, horizontal crustal extension (Wenrich, Billingsley, and 

Huntoon, 1986).

As already mentioned, secondary opening of scattered joints to create prominent 

fissures was noted at many places. Amounts of opening of 0.5-3.0 inches (1.3-8 cm) are 

common among 31-33 joints, but openings of more than 0.2 inch (0.5 cm) are rare among 

the much younger 35 joints, most of which retain their original narrow apertures. The 

probable manner and time of opening of the 32 and 33 joints was mentioned above, but 

the time of opening of the earlier 31 joints is only speculative. If they too were reactivated during 33 time, as seems likely, then a component of left-lateral shear should have 

occurred along the affected joints during reopening. Additional reopening almost 

certainly took place during the late Cenozoic E-W crustal extension of the region, as the 

31 joints are ideally oriented for such to have occurred. Regardless, it is clear from field 

evidence that the large wall separations observed along some 31 joints represent not 

their original apertures, but rather some later event that postdates formation of the 31 

set. Widths of the translucent, white calcite that filled some 31 joints show that their 

apertures at the time of filling were modest, about 0.04-0.08 inch (1-2 mm). Some of 

those filled joints were later cracked open, so that bits of the calcite layer now cling to 

one joint face and the remaining bits to the other. The intervening space, locally as 

much as 3 inches (8 cm) wide, commonly is filled with caliche of quite different appearance from the earlier calcite.

STRESSES INFERRED FROM JOINT SETS

Regional stress histories in the Grand Canyon region most commonly are inferred 

from the movement histories of various sets of faults, but joints offer a supplemental and 

likely more sensitive source of information about more subtle aspects of paleostress 

fields. The reasons are that extension joints in most rocks form at much lower stresses 

than those necessary to induce shear failure in the same materials, and the amount of 

strain necessary to produce even a prominent set of joints commonly is only a fraction of 

one percent. A set of joints, then, often is the ONLY structural record of a given period 

in a region's geologic history. The main disadvantages of joints are that their absolute 

ages can only rarely be determined with certainty, and the causes of the minor strains 

and low stresses that produced them are frequently obscure. Nevertheless, used in 

conjunction with other, more conventional evidence, joints provide a much more thorough 

view of regional stress histories than could otherwise be obtained. As in the preceding 

section, we offer here only a few observations with brief interpretation of the results.

31
Figure 15. Minor normal offsets along reactivated 33 joints near station RM-2, Ridenour 

mine. Faulted dark layer is a mineralized ring fracture. View to WNW.

For extension fractures such as those of the 31 through 35 sets, the orientation of 

some components of the stress field at the time of failure can be confidently inferred 

from the orientations of the joints themselves. Experimental evidence bearing on the 

propagation paths of actively growing extension fractures in rock relative to applied 

macroscopic stresses were summarized recently by Barton (1983). The results show 

consistently that extension fractures propagate perpendicular to the direction of least 

compression (or greatest tension if one or more of the principal stresses are tensile). The 

orientation of o^ 9 then, can be uniquely defined for each joint set in the Esplanade 

Sandstone as the perpendicular to the joint planes. Where a given set exists over a wide 

geographic area as a common element of the total fracture network, the 01 orientation of 

the regional stress field during the time of formation of that set can be regarded as 

comfortably established. Knowledge of the relative ages of the various sets then 

provides some impression of how the stress field evolved through time. The orientations 

of 0[ and (72 are more troublesome, for generally it is known only that they must lie 

within the plane of the joint. In the absence of extreme topographic relief, an oftenmade and frequently valid assumption is that one of the principal stresses must be 

vertical and due to superincumbent load, and the other horizontal. This cannot always be 

demonstrated, however, and thus it is generally more wise to speak of o^ t the vertical 

stress, and Ohmax> tne maximum horizontal compressive stress. The latter is parallel to 

joint strike and is equivalent, in tectonic parlance, to the "direction of crustal (or 

tectonic) compression" or (both vaguely and incorrectly) the "direction of tectonic 

stress". For both, of course, the stress magnitudes are such that ^-0v

32
31 through 33 sets

The existence within the Esplanade Sandstone of at least five sets of extension 

fractures, every one of very different orientation from the rest, shows at a glance that 

the regional stresses changed markedly in direction with time. The first three sets of 

joints, all of which formed under considerable overburden load before canyon-cutting 

commenced, are vertical. The orientation of c^, then, during the (presumably) long time 

interval spanning these three periods of fracture, remained horizontal throughout but 

rotated to different directions with time. With reference to Figures 6, 8, and 9, those 

directions were about N85W (Jl), N20W CI2), and N30E (13). The corresponding directions of "tectonic compression", or %max, during these same periods were about N5E 

(Jl), N70E (12), and N60W (13). All three sets at present can be dated only as post-late 

Triassic and pre-Pliocene, but future work along nearby monoclines and the many faults 

of the region could do much to establish the ages of these larger structures relative to 

the various sets of joints.

Crude estimates of overburden stress during the Jl through 13 fracture episodes 

can be derived from available data. An estimated average specific gravity of 2.49 for 

the section between the Esplanade Sandstone and the top of the Kaibab Limestone can be 

obtained from the lithologic descriptions and thickness data of Wenrich, Billingsley, and 

Huntoon (1986) and the physical properties of different rock types compiled by Balmer 

(1953). Inclusion of the overlying Moenkopi Formation reduces this average to 2.43. 

Using the estimates of overburden thicknesses at the times of fracture from p. 26, 

formation of the Jl through 13 sets occurred under minimum total vertical loads (Sv) of 

about 140 bars (14 MPa). Maximum likely vertical loads were about 230 bars (23 MPa) 

but could have been greater if any post-Moenkopi strata once covered the area and one 

or more of the sets formed during maximum burial. These values, 140-230 bars, are total 

vertical stresses and make no assumptions about fluid pressures during fracture. 

Assuming fluid-saturated rock and no overpressure during fracture, the effective vertical 

stress ((Ty = Sv - p^) would reduce to 85-134 bar (8.5-13.4 MPa). Fluid overpressures 

during any fracture event would, of course, have reduced the vertical load still further.

34 and 35 sets

Late Cenozoic regional uplift initiated a still-continuing cycle of erosion that 

ultimately stripped thousands of feet of overburden from the Esplanade Sandstone. 

Toward the latter part of this period, probably in early Pliocene time, sufficient rock had 

been removed that the vertical was now the o^ direction, and the subhorizontal unloading 

joints of the J4 set developed. Concerning horizontal stresses during this period we know 

only that a %max direction of roughly N-S is compatible with the tectonics of the time 

as recounted by Huntoon (1974) for the nearby eastern Grand Canyon region. The 

Ridenour mine strata during formation of the 34 set almost certainly were within 1,500 

ft (450 m) of the surface, and probably much less. This corresponds to overburden loads 

of less than about 35 bars (3.5 MPa) a considerably different environment from that 

postulated above for the 1\-13 sets. Despite the fact that the strata were already near 

the surface and that incision of the Esplanade Sandstone by the rapidly deepening Grand 

Canyon probably occurred less than a million years later, an additional and final set of 

vertical joints J5 developed. The 15 set shows that crj was once again horizontal and 

probably reflects active tension associated with post-Laramide Basin-and-Range crustal 

stretching in areas to the west.

33
SUMMARY AND CONCLUSIONS

The joint network of the Esplanade Sandstone near the Ridenour pipe is the 

combined product of five separate episodes of extension fracture. Evidence to be 

presented later, from geologic relations in the Ridenour mine, suggests that all five sets 

are of post-Late Triassic age. The first three sets, with average strikes of about due N 

(31), N70E (32), and N60W (33), formed before bed-parallel parting joints (34), due to 

erosional unloading in late Cenozoic time, had split the thick sandstone beds into thinner 

slabs. The 31 through 33 joints thus probably predate the main phase of regional uplift 

and erosion or occurred during its early stages, and formed under conditions of considerable overburden load. Estimates of overburden thickness during 31-33 joint formation 

range from 1,900-3,200 ft (530-950 m), and possibly more. These joints probably 

remained well below the local water table for much of their history and served as 

effective conduits for the movement of fluids, at least some of them reducing. Rock 

immediately adjacent to the joint walls was altered by prolonged fluid flow and 

commonly became further indurated by the precipitation of calcite in available pore 

space. Additional calcite formed coatings on 31 through 33 joint walls, but the comparative rarity of joint intersections, as opposed to simple terminations, shows that complete 

closure of joints by cementation was only locally achieved. The fracture network is well 

interconnected and probably remained open to fluid flow for most or all of its existence.

Regional uplift of between 2 and 3 miles (3.2-4.8 km) during much of Cenozoic time 

resulted in erosional unloading and the attendant formation of bed-parallel parting joints 

of the 34 set, analogous to "sheeting joints" in many plutonic and some metamorphic 

rocks. Additional vertical joints of the 35 set, of average N35E strike, then formed 

within the overlapping slabs of rock bounded by the horizontal 34 joints. Both sets are of 

young geologic age probably early Pliocene and most likely are present only within 

near-surface rocks. Rock adjacent to 34 and 35 joint walls is unaltered and only locally 

coated with thin films of calcite, showing that these young, shallow joints played only a 

minor role in the hydrologic history of the region.

GEOLOGY OF THE RIDENOUR MINE 

GENERAL CONFIGURATION OF PIPE AND MINE

The Ridenour pipe crops out within an east-facing, semicircular embayment high on 

the southwest wall of an unnamed, northwest-trending tributary canyon south of the 

Colorado River (fig. 16). Erosion has cut obliquely through the pipe such that the ringfracture zone along its north, west, and south edges is splendidly exposed in outcrop at 

mine level, and also along the canyon wall below it, but much of the eastern third of the 

pipe has been removed. Ring fractures bordering the east edge of the pipe, however, are 

exposed along the rock-floored stream valley below the mine, and a small outcrop of the 

core breccia occurs on the west side of the same valley, just above stream level, at the 

base of the mine dumps. The rest of the pipe core and the innermost portions of the 

ring-fracture zone are covered by dump material and the access road to the mine, but 

good exposures occur underground.

The pipe in map view is nearly circular (fig. 17). From edge to edge of the ringfracture zone it measures slightly more than 500 ft (150 m) across. The diameter of the 

pipe core, in the only place where this could be measured, is 290 ft (83 m). The latter 

measurement, however, is approximate only, for pipes are known to vary in diameter 

with depth, and the measurement was made of necessity between two points 140 ft (42 

m) apart vertically. The pipe is conspicuous on aerial photographs and is visible from 

afar because the normally red Esplanade Sandstone is strongly bleached to ivory and buff 

colors in and near the pipe. ___________ ___ __ ___ ____ _____

34
Figure 16. Aerial view, looking SSW, of Ridenour mine. Prominent joints in sandstones 

above the mine are members of the 33 set. Photograph by Karen 3. Wenrich, 1982.

The mine workings, most of which are underground, are almost wholly confined to, 

and closely follow, the curving ring-fracture zone within country rock along the western 

edge of the pipe (fig. 17). The main, and lowest, level of the mine, an arcuate drift 

several hundred feet long and about 50 ft (15 m) below the surface, provided access to a 

series of narrow overhand stopes along heavily mineralized portions of the ring-fracture 

zone. Some waste rock was removed through a haulage adit whose portal opened to the 

canyon to the east, and the remainder was used as backfill in several of the stopes, 

rendering them inaccessible. Shallower stopes, also along the ring-fracture zone, were 

worked either downward from access holes at the surface or laterally from adits driven 

into the hillside at road level. A few short prospect adits driven across the ring-fracture 

zone complete the surface workings. Waste rock from the surface excavations was 

dumped into the canyon to the east, nearly covering the portal of the haulage adit from 

the deeper levels.

Within the canyon below the Ridenour mine are four prospect adits, two of them 

driven into the southern part of the ring-fracture zone, one into the northern part, and 

one into the core breccia. The southern adits were both sited along conspicuously 

mineralized portions of the ring-fracture zone, where Cu- and V-mineralized fractures, 

green and charcoal gray in outcrop, cut bleached Esplanade Sandstone. The upper adit 

extends inward only about 10 ft (3 m) to a point where it is clogged by rubble, but the 

lower adit is about 105 ft (32 m) long as measured by pacing. The small adit in the core 

breccia farther north, near stream level, is only about 20 ft (6 m) long and reveals no 

trace of ore minerals. The fourth adit, an oblique cut through the northern edge of the 

ring-fracture zone, is about 60 ft (18 m) long but shows only sparse signs of 

mineralization in the form of small and dispersed spots of malachite. Radioactivity 

counts in all four adits are low, and it is doubtful that any of them produced anything but 

small amounts of copper ore.

The map of Figure 17, adapted from Miller (1954) but with considerable additions, 

shows most of the workings of the Ridenour mine. Although some ore was produced 

during the early 1960's, the mined area today is not much changed from the time of 

Miller's visit.

35
Figure 17. Plan-view map of Ridenour mine, showing surface and underground 

workings. Adapted from Miller (1954) but with considerable additions. Elevations 

shown as solid squares are based on assumed elevation of 4,480 ft at no. 1 adit. Open 

squares denote approximate elevations determined from topographic map.

PRODUCTION HISTORY

Broadly curved ring-fracture surfaces coated with malachite and azurite are still 

preserved in the canyon below the Ridenour mine and doubtless attracted the attention 

of early prospectors, who reportedly discovered the deposit sometime during the 1870's 

(Miller, 1954). Records of early production are scant, but mining for copper probably 

began as early as 1887 (Chenoweth, in press) and continued intermittently until about 

1916. Toward the latter part of this period, during World War I, about 1,000 tons of 

hand-sorted ore was transported by burro and freight wagon to a railhead near Peach 

Springs (Miller, 1934). Thereafter, for more than forty years, the mine lay idle. ______

36
The Ridenour mine attracted attention once more in the 1950's as a potential 

source for uranium. R.D. Miller, then with the Atomic Energy Commission, confirmed in 

his 1954 report the presence of high-grade uranium ore and was guardedly optimistic 

about reserves, stating that uranium ore occurs "in significant quantities" but that the 

uranium-vanadium zone had not yet been exposed sufficiently by the mine workings to 

reliably estimate reserves. Mining records show that only a single shipment of uranium 

ore, totaling 14 tons and averaging 0.15% L^Og and 2.3696 V 2C>5, was made from the 

Ridenour mine in early 1961 (Chenoweth, in press). A small (10 ton) shipment of copper 

ore was also made from the mine about this time. From then on, apparently, the mine 

lay idle once again.

The Ridenour pipe was drilled in 1976 to assess its potential for uranium at depths 

below those exposed by past mining operations. Three holes drilled by Western Nuclear, 

Inc., to depths ranging from 324 to 990 ft (99 to 302 m), disclosed little mineralized rock, 

and in 1978 the company's lease on the property was dropped (Wenrich, Billingsley, and 

Huntoon, 1986). Copper-, vanadium-, and uranium-mineralized rock is nevertheless 

abundant both in the surface and underground workings of the mine, and the site offers a 

splendidly exposed view into the ring-fracture zone of a mineralized breccia pipe.

A detailed review of the historical development of the Ridenour mine is given by 

Chenoweth (in press). ______________

HOST ROCKS NEAR MINE

Beds of the Esplanade Sandstone near the mine commonly are 6-20 ft (2-6 m) thick 

and crop out as a stepped sequence of elongate ledges and low cliffs (fig. 16). The local 

section is 90-95% sandstone, interlayered in places with thin and generally poorly 

exposed layers of siltstone and silty shale. Individual sandstone beds are lenticular in 

cross section, and though most can be traced laterally along canyon walls for hundreds of 

feet or more, many can be seen to pinch out gradually to zero thickness. Nearly all 

contain prominent sets of planar cross strata. Most of the sandstone beds are quartzrich, porous, and only moderately indurated, with variable but generally small amounts of 

calcite as the most obvious cementing agent. Their normal color on both fresh and 

weathered surfaces is pale to medium brick red, except in and near the mine where the 

rock is bleached. A partial stratigraphic section, measured in unaltered strata along the 

east-facing canyon wall just north of the mine, is shown in Figure 18 and described in 

Appendix A. The section includes all units exposed at the surface and in shallow 

workings of the mine.

STRUCTURE OF THE PIPE CORE

The central, brecciated core of the Ridenour pipe is best visible along a haulage 

adit once used to convey waste rock from subsurface workings of the mine eastward to 

the canyon into which the rock was dumped. The adit portal has been blocked and is now 

almost buried by additional waste rock dumped from the higher, surface workings of the 

mine. Present access to the core is via an inclined shaft, partly choked with rubble, 

adjacent to the main entry adit to the mine at road level (fig. 17).

Within the haulage adit the contact between country rock and collapse breccia of 

the pipe core is abrupt and of steep dip. The breccia is polylithologic and consists mainly 

of sandstone and subordinate siltstone derived from the upper half of the Esplanade 

Sandstone and overlying Hermit Shale. Bedding is visible in many clasts and shows that 

they are diversely oriented within the breccia. The breccia is clast-supported and poorly 

sorted, with clast sizes ranging from small pebbles barely an inch (2.5 cm) across to large

37
Unit Thickness 

No. in feet ...

(II) 19

(10) 3 rr-_^_T^-^_ ^_ ^_ .1^- ^i i_i ^ Highest workings of

Ridenour mine

(9) 14

(8) 3

(7) 2.3 

(6) 1.8"

(5) I 2 

(4) 3

(2) 26 "'"".."'"'..."".. ........ .7 ..., ...

Lowest surface workings of 

 '.'.'... ''.... ".'.'A Ridenour mine (no. 3 adit)

jgiiis&

(I) 3 """

Figure 18. Partial stratigraphic section along west wall of canyon immediately north of 

Ridenour mine. Description in Appendix A.

38
slabs more than 10 ft (3 m) in length. Some of the smaller fragments are equant to 

subequant, but the larger clasts occur mostly as plates and slabs with broad upper and 

lower surfaces defined by bedding. Spaces between clasts are filled with loose, white 

sand so that few macroscopic voids remain. The sandy matrix probably was derived in 

large part from disaggregation of sandstone clasts originally cemented by calcite. The 

breccia is unstable and the clasts poorly lithified due to this same process of decementation.

Two aspects of the core breccia are worthy of special note. First, the presence of 

clasts derived from the Hermit Shale shows that downward movement of at least some 

clasts in the Ridenour pipe was in excess of 500 ft (150 m), the approximate vertical 

distance between the haulage adit and the base of the Hermit Shale in nearby 

exposures. This amount of downdrop is well within the range of clast displacements 

reported for other pipes. Second, the character of the core breccia is in full accord with 

a derivation by collapse. Inequant clasts, especially large plates and slabs of thinly 

bedded sandstone and siltstone, tend to lie with their broad, flat surfaces at only shallow 

angles to the horizontal. Only among the smaller and more equidimensional clasts are 

steep as well as shallow dips common. This correlation of dip with clast shape and size is 

exactly what should result from free-fall of debris detached from the walls and roof of 

an open chamber onto a rubble-strewn floor below._________________________

STRUCTURE OF THE RING-FRACTURE ZONE

Ring fractures

Large, arcuate, outward-dipping ring fractures form a well-defined zone within 

bleached country rock around the core of the Ridenour pipe. The structure of this zone 

along a hypothetical radial section from the edge of the pipe core outward is shown in 

Figure 19, and Figure 20 shows some of its properties in map view. The total width of

Figure 19 (next page). Composite sketch showing idealized cross-sectional geometry of a 

ring-fracture zone bordering a breccia pipe in the Esplanade Sandstone, based on 

exposures in the Ridenour mine. Note that the width of the ring-fracture zone varies 

from bed to bed as a function of lithology. The width of the zone of bleached rock 

varies accordingly. Large pockets of mineralized breccia are relatively abundant in 

permeable sandstones cemented primarily by calcite (middle of figure) but become 

less abundant and smaller in less permeable, more firmly lithified beds, especially 

those cemented by quartz (top of figure). Solution cavities along the ring fractures 

locally end abruptly along bedding-plane contacts with less-permeable beds, as at

(A). Breccia within the solution cavities generally is monolithologic and derived from 

the adjacent wall rock, but a few cavities close to the pipe core contain mixed clasts

(B). These clasts, once part of the core breccia, apparently tumbled or were washed 

into large voids along ring fractures that opened directly to the pipe core. The 

core/country-rock contact is steep in most places but locally has a more shallow dip 

where the country rock fell away along a ring fracture, as at (C).

Individual ring fractures in sandstone terminate against major lithologic discontinuities such as the thin shale bed at (D), but commonly extend through the contact of 

one sandstone bed with another (E) of similar lithology. Average dips of ring 

fractures range from 45°--55° for most beds observed, but dips along portions of some 

irregular fractures locally are as shallow as 20° (F) or as steep as vertical (G). Within 

any one bed the ring fractures generally become both smaller and less abundant as the 

outer edge of the ring-fracture zone is approached.

39
WMSS V *fY ^f- \J'- v'u; r 

3:ftfyty--o¥<?.V^ 

. :^:'^&<6^ti&::L.<>.

LEGEND

:

Ring fracture 

r ith solution void 

country rock

:H>w-. Pipe ~ core

T^'/I'-::O» breccia

10 

_j ~\

3

20 ft.

6m

Figure 19

40
. \ Outer m»rg»n of 

^ \C btoach-xJ rock

Outer edge of 

 t -4510', along road

^Out.r edge of ble>ch.d 

rock «long roid

Figure 20. Map showing orientations of ring fractures, pipe-core boundary, and outer 

limit of bleached rock in the Ridenour mine. Triangle symbols = pipe-core breccia; 

stipple = Esplanade Sandstone in ring-fracture zone around pipe. Wavy lines show 

approximate position of ring-fracture zone in canyon below main mine workings.

the ring-fracture zone at any point is difficult to estimate because little of the pipe core 

is exposed at the surface, but a partial width of about 70 ft (21 m) was measured along 

the south end of the pipe. Within this zone are more than 60 subparallel ring fractures. 

A partial width of 130 ft (39 m) was measured on the 4,428-ft level of the mine, from the 

edge of the pipe core to the outer rib of the large arcuate drift to the west. In some 

other places, however, as along the stream valley below the mine and north of adit no. 7, 

the ring-fracture zone probably totals no more than 40 ft (12 m) in width. The mine 

workings for the most part exploit only the most heavily mineralized portions of the ringfracture zone and hence are of limited use in defining its extent, but minimum widths of

41
50-60 ft (15-18 m) appear characteristic of most of the pipe. These widths are to some 

extent controlled by lithology, as it is apparent from observations along both the north 

and south edges of the pipe that the ring fractures within different beds extend outward 

for different distances from pipe center.

Excepting variations in width, the geometry of the ring-fracture zone shows little 

change from the highest surface exposures to the deepest levels of the mine, a vertical 

span of about 80 ft (24 m). Ring fractures are present in abundance along the entire 

preserved extent of the pipe boundary and curve markedly over short distances to strike 

everywhere nearly parallel to the pipe core (fig. 20). Their dips range from about 25° to 

nearly vertical, but these extremes are rare; most of the ring fractures dip between 40° 

and 60°, and the averages for all four stations where these features were measured lie 

between 45° and 53°. Individual ring fractures are large, irregular, and arcuate. Many 

can be followed in continuity for distances of 35-70 ft (10-20 m) along strike and 15-30 ft 

(5-9 m) along dip. Their irregular surfaces, some of which split and merge in complex 

fashion where the ring fractures are closely spaced, contrast markedly with the generally 

smooth, planar to subplanar, and mutually parallel surfaces of nearby sets of tectonic 

joints (figs. 21 and 22).

Spacings of ring fractures vary widely (fig. 23). Most fractures within the main 

part of the ring-fracture zone are spaced less than 20 inches (50 cm) apart, and in some 

places as many as seven of them are present within a band only 8 inches (20 cm) wide. 

Toward the periphery of the ring-fracture zone, however, the spacings generally increase 

and the sizes of individual fractures decrease.

Details of the original topography of ring-fracture surfaces have been obliterated 

in most places by solution of the fracture walls, pervasive mineralization, and subsequent 

oxidation of the minerals to new phases, thereby destroying most direct evidence of 

mode of failure. Although the rock still tends to split along the mineralized ring 

fractures during blasting and excavation, close inspection generally reveals that the 

induced fracture surfaces are not exactly coincident with those of the original ring 

fractures. The search for such delicate features as plumose structure well preserved on 

many joint surfaces in the surrounding country rock is futile in such areas. Nevertheless, two other aspects of ring fractures serve to confirm their identity as extension 

fractures. The first is the phenomenon of "hooking" the tendency of an extension 

fracture during growth to curve toward a nearby free surface, such as another ring 

fracture, to terminate against it at a high angle (Kulander and others, 1979). Welldeveloped hooks of one ring fracture toward another are apparent at scattered points in 

the mine. The second, more telling (and related) feature is twist hackle, wherein a single

fracture surface breaks during growth into a series of overlapping, en echelon segments, 

each of which twists during further growth to increasingly greater angles to the main 

fracture face. The mechanics of twist-hackle formation have been addressed by 

Kulander and others (1979), Pollard and others (1982), and Barton (1983), and will not be 

repeated here; the presence of this feature, however, is diagnostic of extensile failure. 

Fortunately for structural geologists, twist hackle is one of the most common and, by 

virtue of its coarse size, readily preserved of all fracture-surface features in rock. 

Prominent twist hackle was noted on ring fractures in several places, chiefly where these 

fractures were only lightly mineralized and the rock still separates along the original 

fracture planes. The presence of such features as hooks and twist hackle confirms 

expectations based upon theoretical considerations that the ring fractures are surfaces of 

extension failure.

42
Figure 21. View WNW along mineralized ring-fracture zone near station RM-1 and no. 3 

adit (left background) of Ridenour mine. Ring fractures dip moderately to the right. 

Steeply dipping, unmineralized fractures along line of sight of observer are members 

of the later J3 joint set.

Figure 22. Close-up view of anastomosing, mineralized ring fractures (upper left to 

lower right) and later, unmineralized joints near station RM-1 of Ridenour mine. 

Late, supergene goethite unrelated to the primary phase of Cu-V-U mineralization 

coats some of the joints.

43
10 2 °

400 Q o fo 100 110 120 130 140 150 160 

Spacing between ring fractures (cm)

Figure 23. Frequency distribution of ring-fracture spacings in the Ridenour m

Number of readings = 82.
Breccia bodies

Two types of breccia can be recognized in the ring-fracture zone. The first is 

volumetrically insignificant and occurs as small masses generally no larger than 1.5 ft 

(0.5 m) in any dimension. These breccias occur only where closely spaced ring fractures 

anastomose as described above, outlining lenticular fragments of rock that commonly are 

broken further along various subsidiary fractures (fig. 24). Although the rock within such 

areas is highly broken and thus has the fragmental appearance of a breccia, the various 

pieces have moved little from their original positions and have little void space or matrix 

between them. Clearly they are not the products of gravity collapse, in contrast to the 

breccia of the pipe core. All gradations exist between relatively intact country rock 

between widely spaced ring fractures and local breccias of the type shown in Figure 24.

The second, more common type of breccia probably comprised much of the copper 

ore once taken from the Ridenour mine. This breccia occurs as podiform masses in 

solution-widened pockets along individual ring fractures. The solution cavities range in 

dimension from small openings no more than a foot (0.3 m) across to large pockets more 

than 19 ft (6 m) long, 16 ft (5 m) wide, and nearly 3 ft (1 m) thick. These miniature, 

inclined caves, abundantly present in the Ridenour mine, constitute one of its more 

interesting features: whatever the chemistry of the early, pre-ore fluids that circulated 

through and around the Ridenour pipe, clearly they were capable of dissolving large 

amounts of carbonate cement from the host sandstone.

The geometry of a representative breccia-filled cavity is shown in Figure 25. Its 

walls are irregular and rounded by solution. The opposing walls would not match if 

pressed together, and bedding surfaces are offset only a fraction of an inch across the 

void, confirming that the wall separation is not due to mechanical dilation. When traced 

in any direction the openings pinch and swell, narrowing locally to zero width. The 

particular void shown in Figure 25 ends abruptly (at B) against a relatively insoluble bed, 

a fairly common occurrence. The breccia filling this and similar voids (fig. 26) clearly is 

of local derivation, with lithology identical to that of the enclosing host rock. The 

clasts, formed by collapse of solution-weakened rock from the walls of the growing voids, 

are diversely oriented within the breccia and are mostly of small size, less than 4 inches 

(10 cm) across. Clasts as much as 2 ft (0.6 m) in maximum dimension are seen locally in 

some of the larger breccia pods (fig. 27), but very large blocks and slabs analogous to 

those in the core breccia are lacking. The sandy matrix within which the clasts are 

embedded probably also is of local derivation and represents the insoluble residue, chiefly 

quartz, of decemented wall rock. Gradual decementation in place of the embedded rock 

fragments left many of them subrounded to rounded and was an additional source of 

matrix sand. Depending in part on the severity of solution, the relative proportion of 

rock fragments to sand is variable from place to place, so that parts of the breccia are 

clast-supported and other parts matrix-supported. The clasts in some of the lenticular 

openings are concentrated toward the footwall, where they apparently came to rest upon 

the rock surface and one another, and are both surrounded and overlain by fine sand that 

fills the remaining space.

Breccia within the ring-fracture zone differs markedly from that of the pipe core 

in that nearly all of it is monolithologic, with clasts derived wholly from the adjacent 

wall rock. Collapse of solution-weakened rock, where it occurred, was minimal, and 

nearly all of the breccia fragments are within a few feet of their original stratigraphic 

positions, in contrast to displacements of hundreds of feet in the pipe core. Only two 

instances of "foreign" clast types were seen in the ring-fracture zone, both in proximity 

to the pipe core. These particular cavities may have been open to the pipe itself (as at B 

on fig. 19), allowing rock fragments from the core breccia to tumble into them to form 

rare pockets of polylithologic breccia.

45
Figure 24. Minor brecciation of sandstone in highly fractured part of ring-fracture zone, 

station RM-1. Breccia clasts have moved little from their original positions.

2 ft

0 20 40 60 cm

Figure 25. Geometry of breccia- and sand-filled solution pocket along ring fracture 

exposed in vertical section at NE end of arcuate drift on 4428-ft level, Ridenour 

mine. Abrupt end of solution pocket at point (B) occurs along contact between 

soluble (lower) and relatively insoluble (upper) beds.

46
Figure 26. Mineralized solution breccia from ore pocket along ring fracture, ^28-ft 

level of Ridenour mine. Note rounding of clasts.

Figure 27. Local breccia filling solution-widened pocket within ring-fracture zone of 

Ridenour mine. This is the largest pod of breccia yet observed in the mine outside 

the pipe core. Location is in stope at SE end of 4458-ft drift below adit no. 1.

47
ALTERATION OF PIPE FILL AND SURROUNDING COUNTRY ROCK

As in other Grand Canyon breccia pipes, bleaching and decementation of the pipecore breccia and adjacent country rock are the earliest mineralogic changes recognized 

in the Ridenour pipe. The bleaching was accompanied by considerable remobilization of 

iron, present originally as microscopic grains of hematite in the red beds of the 

Esplanade Sandstone and overlying Hermit Shale. Both processes were pervasive and 

affected nearly all of the rock in the pipe core and surrounding strata in contact with it, 

in contrast to the later and more localized effects of mineralization.

Pipe-core breccia

Most of the collapse breccia occupying the core of the Ridenour pipe is white, pale 

gray, or pale orange due to bleaching of the originally red clasts. The most prominent 

exceptions are found low on both ribs near the east end of the haulage adit, where clasts 

of nearly impermeable Hermit Shale retain their original, deep-red color. Some clasts of 

the Esplanade Sandstone in the same area retain vestiges of red color also, but elsewhere along the entire remaining length of the haulage adit and in outcrop they are 

strongly bleached. No attempt was made during this study to document the mineralogic 

changes accompanying bleaching of any of the Ridenour breccias, but Hoffman (1977), 

Sutphin (1986), and Wenrich (1986a) have shown for other pipes that similar bleaching 

involves the destruction by reducing fluids of original finely divided hematite pigment, 

and its removal in solution or local redeposition as another phase, such as pyrite or 

goethite. The chemistry and mineralogy of bleached versus unbleached rock are otherwise similar, except where overprinted by the later effects of mineralization.

The matrix sand of the core breccia and the outermost parts of some of the 

sandstone clasts are in places moderately to heavily pigmented by limonite and, less 

commonly, by secondary hematite. Both minerals locally are distributed in layers of 

varying richness, lending to the matrix a banded appearance and to individual clasts, 

when broken open, the impression of concentric structure. It seems reasonable to assume 

that these iron minerals were derived from the original hematite fraction of the rock, as 

suggested for other pipes, although direct proof is lacking. Whether any intermediate 

phase such as pyrite was involved is unknown. No pyrite was observed in the core 

breccia, although Miller (1954) found it in moderate abundance in the ring-fracture zone, 

and traces of pyrite were later found in drill core from depths of 715-990 ft (214-295 m) 

during a 1978 evaluation of the Ridenour pipe (Wenrich, Billingsley, and Hun toon, 1986). 

The occurrence of hematite as curving, diffuse layers resembling Liesegang bands 

through the Ridenour breccia suggests that it was derived by dehydration from earlierdeposited goethite.

Early dissolution of carbonate apparently was also widespread within the Ridenour 

pipe. Although unaltered strata of the Esplanade Sandstone are not notably rich in 

carbonate cement, the pipe breccia in some places is even less so. It is not uncommon to 

find, within the haulage adit, areas where the clasts and enclosing matrix show no 

reaction to dilute HC1. Removal of carbonate and resultant disaggregation of the 

sandstones, most of which were only weakly or moderately cemented to begin with, likely 

was the source of most or all of the sandy matrix of the core breccia. The same process 

of disaggregation resulted in partial rounding of some of the breccia clasts, a characteristic property of core breccias from other pipes in the Grand Canyon region (Watkins, 

1975; Sutphin, 1986).

Ring-fracture zone

Sandstone and siltstone beds of the Esplanade Sandstone are bleached around the 

Ridenour pipe to an extent controlled by permeability. The bleached rocks are variously

48
pale buff, ivory, pale pink, or grayish tan in color, as opposed to the predominant brick 

red of unaffected rock. The transition from highly bleached to apparently unaltered rock 

is relatively abrupt, taking place over distances of 6 ft (2 m) or less in most units. Beds 

that show bleaching for the greatest distances from pipe center are weakly cemented 

sandstones of such high permeability that a drop of water placed upon a freshly broken 

surface is almost immediately absorbed. With decreasing grain size and increasing 

degree of cementation, permeability and the effects of bleaching diminish. Some wellcemented siltstone beds (unit 7, for example) remain incompletely bleached everywhere 

within the mine area, even in places less than 30 ft (9 m) from the estimated outer 

margin of the (now-eroded) pipe core. In such areas it is not uncommon to see dark red 

to maroon, seemingly unaltered siltstones overlying completely bleached and nearly 

white sandstones.

At the outcrop scale, one can observe at many places in the mine that rock immediately adjacent to the ring fractures is more highly bleached than that further distant. 

Zones of completely bleached rock flanking the ring fractures in the sandstone beds 

commonly range in width from a fraction of an inch to 2.5 inches (6.5 cm). Analogous 

zones are notably thinner in the intervening siltstone layers. The bleaching fluids moved 

through the permeable sandstone beds by a combination of intergranular and fracture 

flow, resulting ultimately in bleaching of nearly their entire mass near the pipe, but with 

the most extreme effects bordering the avenues of greatest permeability the ring 

fractures. Intergranular flow through the intervening mudstones, however, was impeded 

by their low permeability. Miller (1954) earlier had concluded that the fine-grained beds 

of the local section acted as partial barriers to fluid flow during alteration and mineralization of the Esplanade Sandstone.

The iron liberated from destruction of the original hematite in the ring-fracture 

zone was redeposited locally, within the bleached rock, as limonite. Large masses of 

prominently banded sandstone, where the bands are defined by alternating limonite-rich 

and limonite-poor rock the so-called "zebra rock" are widespread in the mine (figs. 28 

and 29). The common parallelism of these bands to nearby ring fractures (fig. 30) 

reaffirms the importance of the ring fractures to fluid flow around the pipe.

As in the pipe core, early fluids moving through the ring-fracture zone removed 

much carbonate from the sandstone beds. Decementation of rock adjacent to the ring 

fractures resulted in formation of the lenticular solution openings previously described. 

Rock fragments and quartz grains freed by solution from the original rock accumulated 

in those openings to form lensoidal masses of breccia that subsequently became mineralized to form part of the copper ore of the Ridenour mine.

PRE-ORE DEPOSITION OF CARBONATE AND SULFATE MINERALS

____Studies of ore samples from other pipes indicate that deposition of carbonate and 

sulfate minerals within the matrix of the core breccia followed bleaching and removal of 

carbonate from the same material, but preceded deposition of the metallic minerals 

comprising the Cu-U ores (Wenrich and Sutphin, in press). Little direct field evidence 

exists for this stage of mineralization in the Ridenour mine, and much of the core 

breccia remains depleted in carbonate. Locally, however within the haulage adit the 

sandy matrix of the core breccia is moderately to highly calcareous, whereas the embedded sandstone clasts are noncalcareous. Inasmuch as the matrix was derived from the 

clasts, calcite apparently was added locally to the matrix at some time following the 

widespread bleaching and early decementation of the rock. Careful examination in thin 

section of calcareous samples would be necessary to determine if this is a pre-ore or 

post-ore effect. However, an early stage of barite deposition in the Ridenour pipe has 

been inferred by H.B. Sutphin (written commun., 1988) from empty crystal molds in a 

sandstone sample from the ring-fracture zone.

49
Figure 28. Well-developed bands of limonite in bleached Esplanade Sandstone at 

Ridenour mine. Bands are parallel to ring fractures (not shown). Note that bands

- pass undisturbed across vertical J3 joints (left side of photo), showing the joints to be 

the later features.

Figure 29. Discontinuous bands of limonite-pigmented sandstone in bleached Esplanade 

Sandstone from Ridenour mine. Dump specimen, 6 inches (15 cm) long.

50
Figure 30. Alternating bands of limonite-rich and limonite-poor rock around terminus of 

a single ring fracture in bleached sandstone near station RM-1, Ridenour mine.

ORE MINERALIZATION

Pipe-core breccia

Copper (and silver?) minerals- Most of the core breccia in the haulage adit and all of it in 

outcrop below the mine is devoid of visible copper minerals. Only two exceptions were 

noted, both within the haulage adit. The first consists of sparse disseminations of 

malachite within matrix sand of the core breccia, very near the outer edge of the pipe 

core. The second, located 35 ft (11 m) from the core boundary, consists of an arborescent coating 3 inches x 5 inches (8 x 13 cm) in area on the external surface of a sandstone clast. The coating is now goethite but preserves perfectly the minute crystal faces 

and herringbone habit of the original mineral, almost certainly native copper, but 

possibly silver. Small amounts of silver have been recovered from some of the Ridenour 

ores, and naumannite (a silver selenide) and bromargyrite (silver bromide) have been 

identified in samples from the mine (Chenoweth, in press). Although copper sulfides and 

sulfosalts are prominent components of the ore from mines in other Grand Canyon 

breccia pipes, and mineralized samples are consistently enriched in silver (Wenrich, 

1986b), neither native copper nor silver have been reported previously.

Uranium and vanadium minerals- No uranium or vanadium minerals were noted anywhere 

within the core breccia, and extensive examination of the entire length of the haulage 

adit with a portable ultraviolet light (fitted with both longwave and shortwave tubes) 

failed to disclose the presence of any luminescent secondary uranium species. Radioactivity counts in the haulage adit (geoMetrics model GRS-101 scintillometer) are uniformly low, from 40 to 70 cps, as compared to background readings of 15 cps in the Esplanade 

Sandstone outside the mine area.

51
The near-total lack of mineralized rock in accessible portions of the Ridenour pipe 

core should not be viewed as abnormal for at least two reasons. First, little of the pipe 

core is exposed, and one cannot assume that those portions currently visible are representative of the whole. Mining in other pipes has confirmed that the distribution of ore 

minerals within the core breccia is irregular. In the Orphan pipe, for example, several 

pods of high-grade uranium ore in the pipe core were separated by large zones of lowgrade to barren material (Magleby, 1961; Chenoweth, 1986). Second, the high permeability of the core breccia and the thorough oxidation of the near-surface portions of the 

Ridenour pipe provide conditions poorly suited to the preservation of the original ore 

minerals. Uraninite, for example, is highly soluble in oxidizing, carbonate-bearing ground 

waters over a wide range of pH (Garrels and Christ, 1965; Drever, 1982) and would 

readily be leached from a permeable rock such as the core breccia of the Ridenour pipe.

Ring-fracture zone

Mineralized rock in the Ridenour mine is almost wholly confined to the ringfracture zone. Detailed study of the various ore and accessory minerals is underway by 

others, and comments here will be restricted mostly to the topic of structural control of 

mineral deposition.

Copper minerals- Throughout most of its history the Ridenour mine was worked for 

copper, contained chiefly in the minerals malachite, azurite, and*chalcocite. These 

minerals occur together in three distinct but related environments. The first is along 

ring fractures, where the copper minerals occur both as open-space fillings and as 

impregnations and partial replacements of the adjacent wall rock. Many of the ringfracture veins are 0.5-1.5 inch (1.3-3.8 cm) thick (fig. 31), and the observed extremes 

range from paper-thin seams to veins 6 inches (15 cm) across, but only a small fraction of 

each vein represents open-space filling. Examination of many veins with a hand lens, 

coupled with later study of mineralized ring fractures in polished section by H.B. Sutphin 

(written commun., 1988), confirms that original openings along most ring fractures were 

small probably no more than 0.04-0.08 inch (1-2 mm), except where widened further by 

early (pre-ore) solution. Such modest openings nevertheless were huge relative to typical 

pore diameters in the adjacent fine- to very fine grained sandstones, and, moreover, 

fluid-flow paths within the fractures were continuous for many meters as compared to 

distances of only one to several grain diameters in the wall rock. Permeabilities along 

the ring fractures thus were orders of magnitude higher than that of the intact rock, and 

the ring fractures became the primary loci for fluid flow and mineralization in the mine 

area. Nearly all of the stoped-out areas in the Ridenour mine are narrow, inclined 

excavations that followed the most heavily mineralized portions of closely spaced ring 

fractures. The degree to which the remaining rock is mineralized shows no notable 

change from top to bottom of the mine, and much copper-mineralized rock remains in 

place.

The lenticular pockets of breccia previously described were a second source of 

copper ore during operation of the Ridenour mine. The sandy matrix of such breccias 

commonly is cemented by copper carbonates (fig. 26), and the same minerals locally have 

partially replaced and impregnated the outer portions of the breccia clasts or formed 

coatings around them. A notable property of such breccia masses is that the most 

densely mineralized rock commonly occurs within a zone 1-4 inches (2.5-10 cm) thick 

along the hanging-wall contact of the breccia with the enclosing rock. One possible 

explanation for this effect is that the reductant necessary for precipitation of the 

original ore minerals was a gas (F^S?) that ponded against the hanging wall during its 

upward migration through the ring fractures (C.S. Spirakis, oral commun., 1988).______

52
Figure 31. Photograph of mineralized ring fractures in bleached Esplanade Sandstone; 

sample from near no. 4 adit of Ridenour mine. Dominant minerals are chalcocite, 

malachite, and azurite.

The third and least significant environment for ore deposition was along bedding 

surfaces that had split open slightly during stoping of the pipe to allow penetration by the 

later mineralizing fluids, forming bed-parallel veins. Despite their orientation in 

common with 34 joints, these veins are much older and date from the time of formation 

of the associated ring fractures. They occur only within the immediate vicinity of the 

pipe and are common only in the fine-grained units siltstones and shales where fissility 

of the original rock presumably aided in their formation. Comparable veins are almost 

nonexistent in the more massive sandstones that comprise the bulk of the section.

Uranium and vanadium minerals- U-V minerals were exploited during the latest (1961) 

phase of mining in the Ridenour pipe, but these are of much more restricted and irregular 

occurrence than the copper minerals. Fine-grained, gray to black vanadium minerals, 

most as yet unidentified, line some ring fractures and impregnate the adjacent wall 

rock. Some veins can be traced as charcoal-gray bands in outcrop for distances of 30 ft 

J9^ m) or more. Olive-green volborthite, a basic copper vanadate, forms conspicuous 

coatings on ring-fracture surfaces in widely scattered parts of the mine. That some ring 

fractures are mineralized predominantly by copper minerals, whereas others are rich in 

vanadium, constitutes one of the more curious aspects of the Ridenour-mine geology.

Visible uranium minerals are not abundant at present in most parts of the mine, but 

radioactivity counts of 100-400 cps in many of the copper-mineralized areas disclose 

their presence in modest amounts. Locally, as near station RM-2, small residual masses 

of dark, vanadium-rich rock encrusted with tyuyamunite and volborthite occur along 

stoped-out portions of the ring fractures and register 5,000-10,000 cps. Tyuyamunite and 

metatyuyamunite (the "carnotite" of Miller, 1954) occur variously as a cementing agent 

in sandstones; as tiny, scattered grains within V-mineralized rock adjacent to ring 

fractures; and in one stope as showy coatings on exposed ring-fracture surfaces. A host 

of other Cu-V-U minerals occur in small quantities in the Ridenour mine but will not be 

described here.

53
Summary

The distribution of ore minerals in the ring-fracture zone shows clearly that ore 

mineralization occurred wherever zones of secondary permeability existed along ring 

fractures, within local masses of solution breccia, and along open bedding planes. Rock 

more than a few inches from such structures generally is bleached but not mineralized. 

Mineralization extended no further outward from the pipe than did the ring-fracture 

zone, and bleaching of the host rock generally extended only 10-20 ft (3-6 m) further.

The nearly complete absence of mineralized rock in the small portion of the pipe 

core currently visible is probably ascribable to thorough oxidation of the highly permeable collapse breccia.

JOINT NETWORK IN MINE

All five sets of joints previously described from intact country rock around the 

Ridenour pipe are also found in altered (bleached) and mineralized rock within the mine 

area (fig. 32). The character of the joint network within the ring-fracture zone and pipe 

core is described below and contrasted with that of the host strata. The various lines of 

evidence uniformly suggest that stoping of the Ridenour pipe, alteration (bleaching) of 

the host rock, and primary mineralization of the ring fractures all predated regional 

fracture of the surrounding Esplanade Sandstone.

Presence of J1-J5 joints within breccia

Most of the solution-collapse breccia of the pipe core, and much of that within 

solution-widened openings along ring fractures, is too poorly lithified to have responded 

brittlely to the same stresses that fractured the surrounding country rocks. Locally, 

however, the breccia is sufficiently cemented by calcite or copper carbonates that 

internal fractures are conspicuous. Those fractures, in all areas examined, can be 

identified as members of the 31 through 35 joint sets. A few examples follow.

(1) Within the haulage adit on the 4,428 ft level, in country rock immediately adjacent to 

the pipe-core breccia, the adit ribs are flat, vertical surfaces of about N70E strike the 

walls of 32 joints. The 32 set here is so strongly expressed that the adit was purposely 

oriented along its strike to take advantage of the ease of excavation in that direction. 

Large, diversely oriented blocks and slabs of sandstone within the adjacent breccia also 

contain internal fractures of about N70E strike and vertical dip. Although fractures 

within the breccia at this locality are not great in number, those seen uniformly conform 

to members of the 32 set.

(2) The outcrop of pipe-core breccia in^the stream valley below the mine is cut by joints 

of the 32 and 34 sets. The breccia here is rather firmly cemented, a fact that 

undoubtedly contributed to its preservation, and appears as a jagged mass of pebble- to 

cobble-sized fragments in a limonite-stained, sandy to silty matrix. Large, irregular 32 

joints that cut uninterrupted through clasts and matrix alike occur in the most indurated 

parts of the breccia. Similar but smaller 32 joints that cut through fewer clasts or are 

confined to single, large blocks are characteristic of less-cemented parts of the same 

mass. The later 34 joints appear as irregular, subhorizontal fractures of 1.5-15 ft (0.5-5 

m) trace length that divide the mass into crude, thick slabs.

(3) A fine example of a breccia pod filling a solution pocket along a ring fracture is 

exposed on the rear face of a stope extending S25E from a point halfway down the 

inclined shaft to the Ridenour mine, at an elevation of 4,458 ft (1,359 m). The breccia 

(fig. 27) is well-cemented by malachite and chalcocite in places and is cut by vertical 

joints of the 31 and 33 sets that extend through both the clasts and intervening matrix.

54
Figure 32. Map showing orientations of Jl through J5 joints in Ridenour mine. Stereonet 

diagrams are equal-area, lower-hemisphere projections. RF = ring fractures.

The evidence from these and similar places consistently suggests that solutioncollapse breccias of the Ridenour pipe were formed and mineralized before jointing of 

the host strata took place, and that both the breccia and surrounding country rock were 

subsequently cut by the same five sets of joints.

Absence of joint-bounded clasts in breccia

If beds of the Esplanade Sandstone were jointed before the Ridenour pipe stoped 

through that formation, then some significant portion of the sandstone clasts in the core 

breccia should have shapes defined in part by those joints. Instead, the only consistently

55
planar external surfaces of the breccia clasts are defined by bedding. Clasts approaching 

the shape of parallelipipeds, with opposite sides parallel and suggestive of joint-bounded 

blocks, are lacking. Similarly lacking are fractures within the clasts that maintain a 

constant angle to bedding regardless of clast orientation, as any set of pre-stoping joints 

must do.

Lack of solution widening along 31-35 joints

Ring fractures within the Ridenour mine contain numerous lenticular caves due to 

solution-widening of the original narrow openings. All details of the original fracture 

surfaces were destroyed as quartz grains were progressively detached from the walls of 

the growing voids during decementation of the rock. Similar solution openings, however, 

are lacking along the J1-J5 joints. The joint walls, both within and outside the mine 

area, often retain delicate features of the original fracture surfaces such as plumose 

structure, except where these have been obliterated by recent weathering. The lack of 

solution effects along the well-preserved J1-J5 joints contrasts strongly with their 

abundant presence along the associated ring fractures.

Terminating relations among fractures

The joints of all five regional sets of the Esplanade Sandstone are extension 

fractures. The ring fractures also formed in extension. As mentioned previously, 

extension fractures generally terminate against older fractures unless those older 

fractures are cemented by some secondary mineral. In that event, the younger fractures 

will either cut across or terminate against the older ones, depending on the degree of 

cohesion between the mineral fill and the wall rock and on the mechanical properties of 

each. All of these effects are well illustrated in the ring-fracture zone.

Ring fractures in the Ridenour mine generally are quite heavily mineralized and 

thus "healed", but in some places, most notably along the south end of the pipe at station 

RM-3, some of the outermost ring fractures are nearly devoid of minerals. At this 

locality, joints of the HI set terminate against various ring fractures, showing that the 

ring fractures predate even the oldest of the five regional sets of joints. Jl joints at 

several places in the mine broke into prominent twist hackle (fig. 33) and curved to 

terminate against a ring fracture at a high angle, both common effects of a younger 

fracture approaching an older one (Kulander and others, 1979). The relative ages of the 

ring fractures and the Jl joint set are especially clear at such places. Comparable 

observations can be made for joints of all other sets at various localities in the mine: for 

each set the component joints either cut across mineralized ring fractures or terminate 

against them, but the converse was not observed.

Size and shape of joints

Joints within the ring-fracture zone tend to be smaller locally much smaller than 

their counterparts outside the pipe because their growth was restricted by pre-existing 

ring fractures. The vertical joints shown in Figures 21 and 33, for example, are only 1.5- 

5 ft (0.5-1.5 m) in height, yet joints of the same sets immediately outside the mine area 

usually are at least several times larger. The ring fractures, in contrast, are of impressively large size, for they formed in previously unbroken rock.

The early presence of the ring fractures also affected the shapes of various joints 

within the ring-fracture zone by introducing irregular and closely spaced anisotropies 

that resulted in local stress perturbations. Whereas many Jl through J3 joints within the 

more homogeneous rock surrounding the pipe are nearly planar, their counterparts within 

the ring-fracture zone are noticeably more irregular.

56
Figure 33. Jl joint showing prominent zone of twist hackle along its termination againstf*^ 

an older, mineralized ring fracture. The younger joint is unmineralized. ' I

Differential mineralization of ring fractures and joints

Although much Cu-V ore in the Ridenour mine was deposited along ring fractures to 

form veins commonly an inch or two thick, joints of the Jl-35 sets in the mine area 

either are barren of ore minerals or contain only sparse, thin coatings of such lateformed supergene minerals as malachite, azurite, and volborthite. The earlier-formed 

sulfides and sulfosalts from the main phase of mineralization, some of which have been 

identified as residual phases in the ring-fracture zone, are lacking entirely in the later 

joints. Coatings of secondary Cu-V minerals on joint surfaces occur only within a meter 

or two (3-6 ft) of heavily mineralized ring fractures, the presumed source of the metals; 

the same joints more distant from the ring fractures remain uncoated. The evidence 

seems clear that primary ores were deposited only within the ring fractures, and that 

secondary minerals derived from them were deposited in small amounts in the later 31-J5 

joints as oxidizing ground water flowed through the pipe.

Zonation of Cu-V minerals near fracture intersections

The simple observation that Jl through 3.5 joints tend to cut across ring fractures 

where these are heavily mineralized, but in many places terminate against them where 

mineralization was weak, strongly suggests that the main phase of Cu-V mineralization in 

the Ridenour pipe predated the formation of any regional joint set. Observations of 

mineral zonation near fracture intersections corroborate that conclusion.

Many ring fractures in the Ridenour mine are mineralogically zoned, the most 

common arrangement consisting of a thin central layer of chalcocite bordered by 

malachite or azurite, or both. The contrasting colors of these minerals lend to many ring 

fractures a pronounced banded appearance in outcrop. The thickness of each band 

commonly pinches and swells along the length of the ring fracture, but there exists no 

relation between such thickness changes and the positions at which the ring fractures are

57
intersected by joints of the J1-J5 sets. Instead, the mineral zones match across each 

joint and show in their form no indication that fluid flow along the ring fractures at the 

time of mineralization was affected by vertical joints. A comparable situation exists for 

those ring fractures mineralized predominantly by vanadium minerals.

Limonite in bleached, jointed sandstone

Limonite, the most widespread noncarbonate gangue mineral of the Ridenour mine, 

occurs commonly in bleached sandstones as a pigmenting agent distributed in layers 

resembling Liesegang bands of exceptional length. The bands in many places are parallel 

to nearby ring fractures (fig. 30) and thus appear to have formed as fluids circulated 

through them. The uniformity and continuity of these bands in the many places where 

they are intersected by joints of the J1-J5 sets (fig. 28) show clearly that the joints were 

not yet in existence during deposition of the limonite. Much of this limonite likely was 

derived from finely divided hematite within the original rock, indicating that considerable remobilization of iron in and around the Ridenour pipe predated the formation of 

any joint set in the area.

Summary

The eight lines of evidence presented above individually and collectively lead to a 

single conclusion that the Esplanade Sandstone was an unjointed unit, structurally 

intact, when the Ridenour pipe stoped through it. Bleaching of the sandstone around the 

pipe, leaching of carbonate from the pipe breccia and surrounding strata, redeposition of 

iron as bands of limonite pigment within massive sandstones, and deposition of primary 

copper, vanadium, and uranium minerals around and within the pipe all occurred before 

the first set of regional joints cut the host rocks.

SUPERGENE MINERALS IN J1-J5 JOINTS

Late Cenozoic oxidation of the Ridenour pipe resulted in such pervasive alteration 

of the primary mineral assemblage that only isolated remnants of the original phases can 

be identified in thin section (H.B. Sutphin, written commun., 1987). Various supergene 

minerals were deposited as patchy coatings on joint surfaces, within a few meters of 

mineralized ring fractures, as oxidizing ground waters leached metals from the ring 

fractures and locally reprecipitated them. Among these minerals are malachite, azurite, 

and volborthite. None of the primary ore minerals the copper sulfides and sulfosalts, 

galena, and uraninite, among others have been found on any joints of the Jl through 35 

sets in the mine. All minerals identified to date from coatings on the joints are compatible with deposition under near-surface, oxidizing conditions.

Three of the minerals identified on J1-J5 joints in the mine area occur beyond it 

also and thus appear to be general to the study area rather than specific to the Ridenour 

pipe. They are described below in their order of deposition on the J1-J5 joints. Their 

presence on the 35 joints, the youngest and structurally most shallow set in the area, 

emphasizes the young geologic age and shallow depth of deposition of these minerals.

Goethite- Dense, black coatings of goethite are common on joints of all five sets 

throughout the Ridenour mine. The identity was confirmed by X-ray diffraction (P.J. 

Modreski, written commun., 1987). These coatings reflect minor redistribution of iron by 

meteoric ground water flowing through the pipe and are of much younger age than the 

widely dispersed, banded limonite referred to above. Goethite locally "bled" into the 

sandstone adjacent to joint walls to form an ochre-colored to brown border, overprinting 

the banded limonite of the earlier generation.________ __ ____________ _____

58
Black coatings similar to those in the mine, but of nearly microscopic thinness, 

were seen in various joints on the Esplanade Platform at places far removed from the 

mine. Their presence probably signifies minor hydration of original hematite in the rock 

bordering the joints, but such coatings, though widespread geographically, are not 

common. Too little material was available for a positive identification, and its correlation in manner and time of occurrence with the goethite in the mine area is only 

assumed.

Calcite- Thin coatings of translucent, cream-colored to white calcite were noted on 

numerous joints of all five sets at many places in the mine. This calcite encrusts the 

goethite deposited earlier on the walls of the same fractures and is visually similar to the 

calcite that fills joints elsewhere on the Esplanade Platform. Its deposition, then, is a 

regional phenomenon unrelated to the Ridenour pipe.

Caliche- As in much of the study area surrounding the Ridenour pipe, caliche in the mine 

area is common in J1-J5 joints and also occurs locally as chalky coatings on exposed rock 

surfaces. Fragments of mineralized rock commonly are embedded in this material where 

vertical joints transect ring fractures near the surface. Caliche also acts as a cementing 

agent of the core breccia, but only within a few meters of the surface, as in the northeasternmost portion of the haulage adit in the mine. The distinctive yellowish-orange to 

bluish-white fluorescence of the caliche is an aid to discerning its presence within parts 

of the breccia and in distinguishing it from the earlier, nonfluorescent calcite cement 

within the breccia matrix.

CHRONOLOGY OF PIPE DEVELOPMENT

A suggested chronology of the evolution of the Ridenour pipe, based on evidence 

presented in preceding pages and on general facts taken from the published literature, is 

given below and summarized in Table 1. Direct evidence for all but the first three 

events is present in the Ridenour mine.

1. Deposition and lithification of the Redwall Limestone in Late Mississippian time.

2. Minor uplift and emergence of the Redwall toward the close of the Mississippian 

Period. Streams cut valleys as deep as 400 ft (130 m) into the Redwall surface. A karst 

topography developed, and the limestone was pervaded by numerous large caverns 1,100- 

1,500 ft (300-450 m) below the future stratigraphic level of the Ridenour mine. Partial 

to complete filling of many of these caverns by fluvial sediments of the Surprise Canyon 

Formation, of Late Mississippian age, establishes their early age (Billingsley, 1986).

3. Possible initial stoping of the Ridenour pipe from Late Mississippian to Early 

Pennsylvanian time. Stoping during this time period of some pipes is inferred from 

observed thickening and sagging of the Surprise Canyon Formation (Late Mississippian) 

and overlying lower beds of the Supai Group (Watahomigi Formation of Pennsylvanian 

age) toward exposed pipes (Billingsley, 1986).

4. Deposition and lithification of the remainder of the Supai Group, including the lower 

Permian Esplanade Sandstone.

5. Renewed collapse and stoping of the Ridenour pipe through the Esplanade Sandstone. 

Resumption of collapse may have occurred in middle Triassic (Chinle) time in reponse to 

the fluctuating water tables of that period (Wenrich, 1986a). The form and large size of 

the ring fractures encircling the pipe core show that the Esplanade Sandstone at this 

time was lithified but unjointed: the pipe stoped through mechanically intact rock.

59
Table ! Chronological development of Ridenour pipe

1 Deposition of Redwall Limestone

2 Uplift of Redwall; caves form

3 Initial stoping of pipes; 

deposition of lower Supai Grp.

4 Deposition, lithification, and 

diagenesis of Esplanade Sandstone

5 Stoping of Ridenour pipe through 

Esplanade Sandstone. 

Ring fractures form

6 Bleaching and decementation; 

reducing fluids

7 Cu-V-U mineralization; 

reducing fluids

8 Jl joint set develops

9 J2 joint set develops

10 J3 joint set develops

11 Regional uplift

12 Oxidation of primary ore

13 Canyon cutting

14 J4 unloading joints develop

15 J5 joints develop

16 Goethite dep. of joints

17 Calcite dep. on joints

18 Dissection of Esplanade 

Platform; drainage of pipe

19 Caliche in all fractures in 

zone of weathering

10

11

12

13

14

Late Late Tr 

Ms Ms P Pe (200-220 Ma) Laramide?

Pliocen

~5 Ma 

Time scale not linear
6. Bleaching and decementation of the core breccia and of strata immediately surrounding the Ridenour pipe. Mineralogic studies of other pipes suggest that iron from 

hematite in the originally red sandstones was redeposited locally by reducing fluids as 

pyrite in the bleached rock (Gornitz and others, 1988). Removal of carbonate from the 

host rocks, probably simultaneous with bleaching, produced numerous lenticular solution 

openings along ring fractures encircling the pipe. Quartz grains and small rock 

fragments freed by decementation of the host rock filled the solution openings and, in 

the pipe core, formed much or all of the matrix of the collapse breccia.

Diagenesis of the Esplanade Sandstone had already been completed by this time, 

because the distance from pipe center that individual beds are bleached correlates well 

with their present degrees of permeability in unaltered strata outside the pipe area.

7. Cu-V-U mineralization within bleached rock in and peripheral to the pipe. Conditions 

at this time were reducing and, by analogy to pipes where reduced ore is better 

preserved, led to the deposition of such phases as chalcopyrite, galena, pyrite, bornite, 

sphalerite, enargite, and uraninite, plus numerous less-common sulfides and sulfosalts 

(Wenrich and Sutphin, in press). Of these minerals, the first four have been identified as 

residual phases in ore from the Ridenour mine, and the abundant chalcocite probably was 

derived from original enargite or chalcopyrite (H.B. Sutphin, written commun., 1988). 

Ludwig and others (1986) have shown from U-Pb age determinations that the main phase 

of uranium mineralization in several other pipes occurred about 200-220 Ma (middle to 

late Triassic). These dates fix a probable upper limit for the Cu-V mineralization of the 

Ridenour pipe, because Wenrich (1986a) and Wenrich and Sutphin (in press) suggest from 

textural evidence of unoxidized ore samples from other pipes that deposition of uraninite 

postdated most of the other ore minerals.

8-10. Formation in succession of the three regional joint sets 31 (N), 12 (N70E), and 13 

(N60W). The actual times of fracture and the intervals between them remain unknown, 

although the 12 set may be of Laramide age. Available evidence suggests that all three 

sets formed at depths of 1,900-3,200 ft (560-975 m), and possibly greater. Removal of 

hematite from rock bordering the joint walls formed narrow bleached zones with common 

widths of 0.1-0.4 inch (0.25-1.0 cm), and shows that fluids circulating through the joint 

system (and thus the Ridenour pipe) were still reducing. Their chemistry clearly had 

changed with time, however, for the joints unlike the earlier ring fractures were 

neither mineralized nor widened by dissolution of carbonate from their walls, which are 

largely intact. The correlation of increasing width of bleached zones with increasing 

joint age shows that all three sets remained open and served to channel ground-water 

flow for an extended period of time.

11-12. Regional uplift and supergene oxidation of the primary ore. Uplift may have 

begun during or shortly after Laramide (Late Cretaceous through Eocene) time and 

continued through much of the late Cenozoic (Wenrich, Billingsley, and Huntoon, 1986). 

Supergene oxidation of the primary ore commenced during uplift as the upper reaches of 

the Ridenour pipe were raised into the zone of oxidizing ground waters. Probable 

acceleration of uplift in late Cenozoic time (Lucchitta, 1979; Wenrich, Billingsley, and 

Huntoon, 1986) prompted Wenrich and Sutphin (in press) to suggest that much supergene 

alteration of the original phases occurred within the past five million years. Minerals 

formed during this time in the Ridenour mine include, in approximate order of abundance, malachite, azurite, volborthite, tyuyamunite and metatyuyamunite, conichalcite, 

and vesignieite. The same minerals were deposited in small amounts on nearby joints as 

metals leached from mineralized ring fractures were reprecipitated locally. Comparable 

coatings of Cu-V-U minerals were nowhere noted outside the mine area. Oxidation of 

primary ore was a protracted process that continues to the present day; it thus overlaps 

in time all of the events listed below. _____

61
13. Initial stages of excavation of the Grand Canyon. Lucchitta (in press) suggests that 

canyon cutting probably began shortly before 5 Ma but may have begun as much as a 

million years sooner.

14. Formation of bed-parallel parting joints (the 34 set) in response to continued regional 

uplift and erosional unloading. Such joints generally form only at shallow depths. The 

presence on nearby plateaus of some 2,000 ft (600 m) of strata above the level of the 

Ridenour mine shows that canyon cutting by 34 time must already have begun, in order to 

lower the local erosion surface to a level where unloading joints could develop in the 

Esplanade Sandstone. An age of about 5 Ma is likely for the 34 set in light of the abundant evidence that much of the lower Grand Canyon was excavated 4-5 Ma (Lucchitta, in 

press). The fresh, unaltered wall rock adjacent to 34 joints beyond the immediate mine 

area confirms that they have not long served as conduits for ground-water flow. 34 

joints in the mine contain only sparse coatings of secondary Cu-V-U minerals derived 

from nearby ring fractures.

15. Formation of the 35 (N30E) joint set, the youngest set known from this area. Like 

the 34 joints, these joints formed at shallow depth and had little impact on paleohydrologic flow. Local coatings of coarse-grained, translucent calcite on joints of both sets, 

however, suggest that they formed below the local water table, before erosional dissection drained this part of the Esplanade Platform. So too does the uniform orientation of 

the 35 joints across the Esplanade Platform: the implied absence of topographic perturbations of the stress field during 35 time shows that canyon incision near the Ridenour 

mine had not yet reached the level of the Esplanade Sandstone.

16-17. Deposition of goethite, followed by calcite, in joints of all five sets. Goethite in 

the Ridenour mine appears as dense black coatings on joint surfaces and probably was 

derived locally, from previously deposited, iron-bearing minerals within the pipe and 

associated ring-fracture zone. Analogous coatings on joints beyond the mine are present 

but of microscopic thinness. White, sugary calcite, however, is common in joints 

throughout the area studied. Calcite of similar appearance in the Ridenour mine 

commonly constitutes the latest phase in residual vugs within breccia-filled solution 

cavities of the ring-fracture zone.

18. Drainage of ground water from the once-saturated Esplanade Sandstone due to 

erosional dissection (canyon cutting) of the Esplanade Platform. Since at least that time, 

flow within the upper portions of the Ridenour pipe has been vertically downward. 

Ground water saturated in carbonate is locally depositing calcite in small amounts as 

stalactites, stalagmites, curtains, and flowstone within the Ridenour mine.

19. Widespread deposition of caliche in joints of all sets, both within and beyond the area 

of the Ridenour pipe, in near-surface environments.

DISCUSSION 

PALEOHYDROLOGIC IMPLICATIONS

Structural relations in the Ridenour mine show that the pipe itself is the oldest 

post-depositional structure known within the study area, and that rocks around the pipe 

were both altered and mineralized before any regional joint set had formed in the 

Esplanade Sandstone. If primary mineralization in the Ridenour pipe occurred about 200- 

220 Ma, as seems likely from U-Pb age determinations in other pipes (Ludwig and others, 

1986), then at least 60 m.y. elapsed between deposition of the Esplanade Sandstone and

62
the development of the first regional set of joints within it. For some unknown fraction 

of that time the Ridenour and kindred pipes nearby were the ONLY macroscopic conduits 

for vertical fluid movement across formation boundaries in the area.

Limited data from other parts of the Grand Canyon region suggest this situation 

was not unusual. The work of Sutphin (1986), for example, suggests that many pipes on 

the southern Marble Plateau (fig. 1) are old structures cut by predominantly younger sets 

of joints. Sutphin noted that many shallow sinkholes in the Kaibab Limestone on the 

plateau surface have rectangular outlines controlled by well-developed sets of joints, 

whereas the older breccia pipes are invariably almost circular. Alteration haloes of 

bleached rock around the pipes are circular also, implying the absence at the time of 

alteration of strong permeability gradients within the plane of bedding, and thus the 

probable absence of fractures.

Immediately south of the area mapped by Sutphin, on the Black Point monocline in 

the Little Colorado River valley 106 mi (170 km) ESE of the Ridenour mine, lies the 

Riverview pipe (fig. 1). This pipe was mined in the 1950's for uranium extracted from a 

mineralized collapse breccia (Chenoweth and Blakemore, 1961). A recent study of the 

structure of the mine and its immediate surroundings (E.R. Verbeek and M.A. Grout, 

unpub. data, 1986) suggests that the regional joint sets there postdate both the Riverview 

pipe and the monocline. The pipe is exposed within middle (?) and lower Triassic strata 

of the Moenkopi Formation and contains within its core several large, downdropped 

blocks of Shinarump Conglomerate (upper Triassic). Both the displaced blocks and the 

surrounding rock are cut by the same two sets of joints. Bed dips along the monocline in 

this area range from horizontal to about 25° NE, but the joints remain everywhere 

almost vertical, suggesting that the monocline is the older structure. With the exception 

of minor thrust faults and associated local fractures related to the development of the 

Black Point monocline in Laramide time, the Moenkopi Formation near the Riverview 

mine apparently remained unjointed for more than 150 m.y.

The recent study of joints on the western Hualapai Plateau by Roller (1987) also 

suggests that some, and perhaps most, of the joint sets there are geologically young. She 

described four sets of joints in the Supai Group (Pennsylvanian-Permian) that appear to 

correlate with similar sets observed within Tertiary basalts and conglomerates. Work 

within the Tertiary units was of limited scope and the suggested correlations thus are 

tentative, but the fact remains that multiple episodes of fracture in that area occurred 

in Tertiary time. New work by Roller (in press) extends similar conclusions to areas near 

the Ridenour mine.

Finally, recent work in the solution-collapse breccia pipe of the Apex mine near St. 

George, Utah, shows that stoping of the pipe, alteration of the wall rock, and mineralization of the breccia predate Late Mesozoic thrusting in the mine area (Verbeek and 

others, 1987). The Apex mine is in the Basin and Range Province and thus in a tectonic 

setting quite different from that of the Grand Canyon pipes, but Wenrich and others 

(1987) argue from geologic and geochemical evidence that the pipes themselves are 

kindred structures. Given that those Grand Canyon pipes studied in detail are known to 

considerably predate structural distinction of the Colorado Plateau from the Basin and 

Range Province, the discovery of a mineralized pipe west of the Plateau was to be 

expected. Again, in that area, the pipe itself is old relative to other nearby structures.

Although no attempt is made in this paper to interpret paleohydrologic flow 

directions and sources of ground water (but see Huntoon, 1970, 1986), the evolution 

through time of structural conduits for ground-water flow in the Grand Canyon region is 

deserving of comment. The combined evidence from the several studies noted above 

supports the tentative conclusion that many of the Grand Canyon breccia pipes were both 

formed and mineralized before pervasive fracture of their various host formations 

occurred. The "parent" formation of those pipes, the Redwall Limestone, was deposited 

in Mississippian time. By the close of that period an extensive system of caverns, some 

of them already filled to various levels by fluvial sediments of the Surprise Canyon

63
Formation, existed throughout the Redwall Limestone (Billingsley, 1986). In contrast to 

overlying formations, which had yet to be deposited, the Redwall Limestone at this time 

may already have been fractured (Roller, 1987, in press). The possibility that initial 

cavern development in the Redwall may have been controlled by joint sets of 

Mississippian age had been raised earlier by Sutphin and Wenrich (1986). If true, hydraulic connection via a fracture network between the Redwall and underlying strata, and 

with the highly faulted crystalline rocks beneath, existed before deposition of the 

overlying strata.

By late Triassic time hundreds, if not thousands, of breccia pipes had stoped upward 

in the Grand Canyon region to various levels within the Pennsylvanian through Triassic 

strata overlying the Redwall Limestone. The post-Redwall strata at this time apparently 

remained unjointed over much of the region, virtually precluding rapid vertical movement of ground water in any place other than through breccia pipes. The pipes were 

mineralized by Cu-V-U-bearing fluids of unknown provenance in middle to late Triassic 

time. The general absence of fault-controlled ore bodies derived from the same fluids is 

an additional indication that the post-Redwall strata were as yet little fractured.

The times at which various regional sets of joints formed within the post-Redwall 

strata generally are unknown, but the bulk of the evidence suggests that most of the sets 

are of Laramide age or younger. Regional integration of the ground-water flow network 

through a pervasive system of fractures is thus a rather young element of Grand Canyon 

geology. Flushing of breccia pipes by oxidizing meteoric water at this time led to 

alteration of the primary ore within the upper reaches of many pipes to form mineral 

assemblages such as that observed today at the Ridenour mine.

STRUCTURAL CONTROL OF BRECCIA PIPES

Apparent alignments of breccia pipes in many areas of the Hualapai, Coconino, and 

southern Marble Plateaus have led to the repeated suggestion that the locations of some, 

and perhaps many, pipes are structurally controlled. However, the nature of the controlling structures and the manner in which they influence pipe development have remained 

obscure. A few pipes the Riverview, for example lie along the tilted flanks of lengthy 

monoclines. Others lie within or sufficiently close to a major fault zone that some 

workers (Osterwald, 1965; Baillieul and Zollinger, 1980) feel a genetic link is implied, and 

still others, such as the Bat Cave pipe (Wenrich, 1985), rest squarely on faults of moderate throw. For many pipes, however, obvious evidence for structural control is lacking, 

at least at the surface. The Ridenour pipe is one of these. With respect to major 

structures, the mine lies 2.5 mi (4.0 km) east of the Hurricane fault zone and 4.0 mi (6.4 

km) west of the Toroweap fault zone. The nearest "major" structure, the Lava fault, has 

a mapped offset of 400 ft (120 m) (Wenrich, Billingsley, and Huntoon, 1986) and lies 1.4 

mi (2.3 km) northwest of the mine. Several minor normal faults with offsets of 10-25 ft 

(3-8 m) occur within a half mile of the mine on its northeast side, but none impinge on 

the pipe itself. The only fault within the mine has a mapped normal offset of 3 ft (1 m) 

and postdates both the pipe and the main phase of mineralization. There is little evidence at the Ridenour mine that any mapped fault influenced the pipe location or 

facilitated stoping, and all five joint sets that cut the host rock are now known to 

postdate creation of the pipe. Similar statements could be applied to the Riverview pipe 

and apparently to many other breccia pipes as well.

In retrospect, the general inability to relate locations of breccia pipes to specific 

structures in the surrounding rocks should not be surprising: the pipes were mineralized 

in early Mesozoic time, whereas the monoclines, faults, and most or all of the joint sets 

that deform the post-Redwall strata are far younger, of Laramide and later age. This, 

however, does not entirely negate the possibility of control by much older structures, 

evidence for which might exist only at deeper and generally unexposed stratigraphic

64
levels where the pipes first started to develop. One possible explanation for structural 

control of breccia pipes is reviewed briefly below for the two areas where it has been 

most frequently invoked.

Marble Plateau

For the southern Marble Plateau (fig. 1), Sutphin and others (1983), Sutphin and 

Wenrich (1986, 1988), and Sutphin (1986) suggested that the lack of coincidence between 

trends of aligned breccia pipes and of prominent fracture sets at the surface was evidence for a deeper level of structural control. They hypothesized that the Redwall 

Limestone was cut by early sets of fractures that controlled initial cavern development 

but that have no expression in the Kaibab Limestone capping the plateau surface 2,000 ft 

(600 m) above. The similarity in trend between pipe alignments (Sutphin and Wenrich, 

1986, 1988; Wheeler, 1986) and known and inferred fault zones in the crystalline basement rocks beneath the Marble Plateau led them to the further inference that minor 

reactivation of those basement fault zones in Paleozoic time was responsible for the 

development of the hypothesized fractures, in linear zones parallel to, and directly 

above, the faults.

Reactivation of Precambrian fault zones is well known from the Grand Canyon 

region and occurred in both normal and reverse senses at various times. Its effects are 

complex, particularly where structures from multiple episodes of movement are superimposed, but all are strongly dependent on the magnitude of slip. Where displacements 

were large, reactivation of Precambrian fault zones commonly resulted in faulting of the 

entire section of Paleozoic rocks above. Long, narrow zones of subparallel faults across 

the plateaus flanking the Grand Canyon are one expression of this process; the Hurricane 

and Toroweap fault zones are familiar examples. Lesser amounts of slip locally resulted 

in faulting of the lower parts of the sedimentary section but not of higher units because 

throw across many of the faults diminishes upward. Huntoon (1974) described such 

structures from the eastern Grand Canyon and attributed the decreasing throw with 

height to ductile deformation within thick sections of weak rock such as the Bright Angel 

Shale. The key point as it relates to the hypothesis of Sutphin and his colleagues is that 

minor reactivation in Paleozoic time of Precambrian faults beneath the Marble Plateau 

conceivably could have resulted in local fracture of the Redwall Limestone but not of 

overlying units, particularly if such reactivation occurred during the minor period of 

uplift and emergence of the Redwall in Late Mississippian time, before the overlying 

units were deposited. If so, lengthy belts of highly fractured Redwall above the basement faults would have been favored sites for initial cavern development and for subsequent stoping to form chains of breccia pipes as seen at the surface. The controlling 

structures, however, would remain "blind" and have no expression in exposed rocks. This 

hypothesis is plausible and fits known facts, but it remains incompletely tested, as 

nothing is known about the fracture network in the Redwall Limestone beneath the 

Marble Plateau. The nearest exposures, in the canyon of the Little Colorado River, are 

nearly inaccessible.

Hualapai and western Coconino Plateaus

A similar hypothesis of structural control has recently been propounded for the 

Hualapai lands, including the area of the Ridenour mine on the western edge of the 

Coconino Plateau, by Billingsley and others (1986), Wenrich, Billingsley, and Huntoon 

(1986), Sutphin and Wenrich (1987), and Wenrich and Sutphin (in press). Opportunity 

exists in this region to address the topic of structural control directly, for the "parent" 

formation of the pipes the Redwall Limestone is exposed and accessible to study in 

many places not far west of and below the Ridenour mine. One intriguing result from 

reconnaissance studies of the regional fracture network by Roller (1987, in press) is that

65
the two earliest sets of prominent joints in the Redwall Limestone on the Hualapai and 

western Coconino Plateaus do not cut overlying strata of the lower Supai Group. Likewise, no counterpart to those early sets was found by us at the level of the Esplanade 

Sandstone around the Ridenour mine. Roller (1987) suggested as one possibility that the 

Redwall Limestone was lithified and then cut by the two early sets of joints prior to 

deposition of the overlying Supai Group. If so, the joints are of Late Mississippian to 

Early Pennsylvanian age and thus could have influenced both early cavern development in 

the Redwall Limestone and subsequent stoping of pipes through the rest of that formation.

Testing of this hypothesis should be relatively straightforward, for if it is true, the 

structure of breccia pipes within the Redwall Limestone should differ markedly from 

those in higher units in at least four ways: (1) Breccia pipes stoped through previously 

broken rock in the Redwall should lack a well-defined ring-fracture zone. Ring 

fractures, if they did form locally, should be small and should terminate against the 

earlier joints unless those joints had already been healed by precipitation of secondary 

minerals such as calcite. Roller (in press) recently addressed this topic and noted that 

ring fractures are absent around pipes that crop out in the Redwall. (2) Individual pipes 

stoped through jointed rock might well be elongated in plan view parallel to the most 

prominent set of fractures, whereas the same or other pipes at higher stratigraphic 

levels, cut only by post-pipe joints, should tend toward circular outlines. In this respect 

it is interesting to note that all but 2 of 93 pipes mapped in Permian and Triassic strata 

of the southern Marble Plateau are circular (Sutphin, 1986), whereas numerous pipes 

mapped by Billingsley and others (1986) in the Blue Mountain area of the Hualapai 

Reservation, immediately above the top of the Redwall, are elongated in a N40-60E 

direction. The direction of elongation is parallel to one of the two early sets of preSupai joints documented by Roller (1987) in the Redwall Limestone. (3) Pipes stoped 

through jointed rock should contain in their cores polyhedral blocks of collapse breccia. 

Roller (in press) noted exactly this in some pipes within the Redwall Limestone some of 

the clasts had planar external faces, defined apparently by joints. (4) Wenrich (1985) and 

Sutphin (1986) noted that many breccia pipes are encircled by inward-dipping beds due to 

solution-thinning of nearby strata. Old, pre-pipe joints in dipping beds should have 

rotated with those beds to new and generally nonvertical orientations, whereas post-pipe 

joints would not be so affected. Close attention to the orientations of various joint sets 

as they are traced from horizontal beds outside pipes to dipping beds near them should 

provide one ready means to determine which, if any, among several or more sets of joints 

were present at the time of pipe formation.

Mapping of aligned pipes and statistical testing of suggested alignments for significance has not yet been accomplished for the Hualapai pipes, nor has it been determined 

if the early (pre-Supai Group) joints mapped by Roller (1987, in press) are related in trend 

and distribution to underlying basement fault zones. Those early joints have average 

strikes of N48E and N51W, almost exactly matching the preferred trends of breccia pipes 

on the Marble Plateau, but so much distance separates the two areas (85-100 mi, or 135- 

160 km) that no special significance should be attached.

IMPLICATIONS FOR MINING OF BRECCIA PIPES

The geometry of breccia pipes and the distribution and grade of Cu-V-U ores within 

them are complex functions of several factors, not all of them well understood at 

present. Factors often mentioned include lithology, structure of the ring-fracture zone, 

the source, chemistry, and temperature of the mineralizing fluids, whether these fluids 

moved upward or downward through the pipes, and the origin, nature, and possible 

migration through pipes of reductants that led to precipitation of the primary ores. 

Some of the geochemical factors are addressed in papers by Gornitz and Kerr (1970),

66
Gornitz and others (1988), Rasmussen and others (1986), Sutphin (1986), Wenrich (1985, 

1986b), and Wenrich and Sutphin (in press). Below we address only a few of the more 

obvious physical factors affecting ore deposition in mineralized pipes.

Structure of ring-fracture zone

The ring-fracture zone in mined Grand Canyon breccia pipes commonly is a site of 

rich concentrations of ore. The width of this zone, and thus the potential volume of 

mineralized rock, varies within relatively narrow limits at any particular stratigraphic 

level but shows marked variations vertically. At the Ridenour mine the width of the 

ring-fracture zone ranges from somewhat less than 40 ft (12 m) near the no. 7 adit to at 

least 130 ft (39 m) on the 4,428-ft level, about HO ft (42 m) higher stratigraphically. 

Kofford (1969) noted for the Orphan pipe that the mineralized ring-fracture zone on the 

365-ft level measures 500 ft (150 m) edge-to-edge around a pipe core of 380 ft (115 m) 

diameter, showing that the ring-fracture zone on that level averages about 60 ft (18 m) 

in width. The 365-ft level is about 230 ft (65 m) below the base of the Hermit Shale and 

thus probably is in the lower part of the Esplanade Sandstone, as is the Ridenour mine. 

The widths of the ring-fracture zone in the two areas are comparable. Within the Hermit 

Shale, however, the ring-fracture zone around the Orphan pipe narrows to 6-20 ft (2-6 m) 

(Kofford, 1969). It is likewise narrow in this unit around the EZ-2 pipe, about 40 mi (64 

km) NNE of the Ridenour mine. Krewedl and Carisey (1986) state that bleaching of the 

Hermit Shale around the EZ-2 pipe extends no farther than 50 ft (15 m) from the pipe 

core; the width of the associated ring-fracture zone presumably is somewhat less.

It seems clear from this and additional published information that lithology exerts a 

strong influence on the dimensions and character of the ring-fracture zone. Similar 

effects are evident at the Ridenour mine on a bed-to-bed level. Maximum widths of the 

ring-fracture zone and the most strongly mineralized rock appear to correlate with beds 

of mechanically weak, highly permeable sandstone, whereas well-cemented sandstones of 

low permeability are distinctly less favorable environments for ore. The strongly 

cemented sandstone of unit 11 (fig. 18), for example, contains no ore, nor is it broken by 

ring fractures, despite the abundant presence of both directly below in the underground 

stopes. The lower contact of this bed marks the uppermost extent of the mine 

workings. Beds of mudstone and shale may be the most unfavorable lithology of all, as 

they are at other pipes (Kofford, 1969; Chenoweth, 1986). Ring fractures within shaly 

layers at the Ridenour mine are poorly developed, as the rock instead tended to split 

along discontinuous fractures parallel to the plane of fissility. Bed-parallel veinlets of 

copper minerals were noted in a few such places, but the veinlets are thin and ore grades 

low. In other places the shaly beds are barren.

Diameter of pipes

Diameters of individual pipes, like the widths of ring-fracture zones, vary from 

level to level, and for the same reasons. Again a rough correlation with lithology is 

evident, as pipe diameters seem greatest in thick units of relatively weak sandstone (e.g. 

the Supai Group) and least in formations dominated by shales and mudstones (e.g. the 

Hermit Shale) or by well-cemented sandstones of exceptional mechanical strength. The 

Orphan pipe, for example, flares from a minimum diameter of about 165 ft (50 m) near 

the base of the Hermit Shale to about 490 ft (150 m) within the underlying Esplanade 

Sandstone, an increase of 325 ft (100 m) over a comparable vertical distance (Gornitz and 

Kerr, 1970). Krewedl and Carisey (1986) likewise noted the dependence of pipe diameter 

on rock type and suggested also that the size of the original caverns in the Redwall 

Limestone may have played some role. The limited exposures of the core breccia at the 

Ridenour mine and the small vertical extent of the mine workings do not allow these 

factors to be directly addressed for that pipe, nor are drilling records sufficient to 

reconstruct its subsurface geometry.

67
Fracture history of host rocks

Knowledge that different stratigraphic units are cut by joints of different geologic 

age in the Grand Canyon area (Roller, 1987, in press) introduces additional potential for 

vertical variability in pipe geometry and mineralization. Nearly all of the Cu-V-U ore in 

the Ridenour mine occurred along large, arcuate ring fractures encircling the pipe 

because these were the only fractures in existence at the time of ore formation. The 

edge of the ring-fracture zone in this mine forms a sharp cutoff of mineralized rock. A 

similar situation likely prevailed at the Orphan mine, which also had a well-developed 

ring-fracture zone within the upper, mined portions of the pipe.

The pattern of mineralization (if any) at deeper levels in these and similar pipes, 

however, may be much different if older sets of joints predating ore formation are 

present in the older rocks. Such joints, if open, likely would have allowed migration of 

the ore fluids to much greater distances from the pipe than was characteristic of the 

higher strata, resulting in a more diffuse pattern of mineralization with no well-defined 

outer boundary. Strata cut by pre-pipe joints may contain only a weakly developed ringfracture zone or be missing one entirely, as around some of the Redwall pipes recently 

examined by Roller (in press). In this regard it is perhaps significant that a series of 

holes drilled in 1961-62 from the lowest levels of the Orphan mine failed to locate 

evidence of a ring-fracture zone in strata much below the mine workings (Chenoweth, 

1986). The age of various joint sets relative to the times of pipe formation and mineralization is an additional factor potentially influencing the character of ore bodies and their 

variation with depth, but one for which little information is available for most parts of 

the Grand Canyon region.

ACKNOWLEDGMENTS

Research for this report was funded by the Bureau of Indian Affairs in cooperation 

with the Hualapai Tribe. Karen J. Wenrich suggested the work and handled the many 

logistical concerns leading to its completion. Hoyt B. Sutphin was a valuable source of 

information and served as a friendly critic of our ideas. George H. Billingsley provided 

the aerial photographs of the mine area, and Julie A. Roller kindly provided information 

on her fracture studies of the Hualapai Reservation in advance of publication. Discussions with all four colleagues over a period of years did much to refine some of the ideas 

presented here and ensured that others would never see print. Reviews by Christopher C. 

Barton of previous drafts of this paper are gratefully acknowledged.

68
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APPENDIX A

Measured stratigraphic section near Ridenour mine

(described from top to bottom)

Unit 11

Noncalcareous to slightly calcareous, quartz-cemented, fine-grained sandstone. 

Color variable; medium pink to salmon pink where most firmly cemented, grading locally 

into brick red and weakly cemented sandstone resembling that of underlying beds. 

Prominently crossbedded in sets 6-10 ft (2-3 m) in height. Unit is 19 ft (5.8 m) thick.

This sandstone is of quite different character from all sandstone beds described 

below. Much of it is strongly cemented and nonporous, in contrast to the weakly 

cemented, locally friable, and highly porous sandstones of underlying beds. On broken 

surfaces this sandstone has a finely frosted appearance, and individual quartz grains are 

difficult to distinguish by eye because of pervasive quartz cement. Other sandstones 

below, when broken, show only dull, rough surfaces with individual quartz grains standing 

in relief.

Unit 10

Moderately to weakly cemented, generally noncalcareous, coarse siltstone. 

Medium to dark brownish red on both fresh and weathered surfaces. Poorly exposed; 

generally found as float or small outcrops below overhanging ledge of unit 11. 3 ft (0.9 

m) thick.

Unit 9

Porous, weakly cemented, very fine grained sandstone. Generally noncalcareous 

but locally moderately calcareous. Color ranges from pale pink or nearly white to 

medium brownish red or brick red on both fresh and weathered surfaces. Mottled 

appearance is common. Locally friable and easily disaggregated by scratching with pick 

end of hammer. Lower third crossbedded in sets 1-1.5 ft (30-45 cm) thick; weathers to 

smooth, rounded forms. Upper two-thirds massive and weathers to irregular, knobby 

surfaces. 14 ft (4.3 m) thick.

Unit 8

Siltstone; dark brownish red on weathered surfaces. Forms covered slopes north of 

mine and is inaccessible in mine area, so was not inspected closely. Rock is considerably 

darker in color than underlying units, and thus presumably is finer grained. Fissile; 

weathers to thin plates and laminae. 3 ft (0.9 m) thick (estimated).

Unit?

Massive, porous, slightly calcareous, medium brownish-red coarse siltstone to finegrained sandstone. Sinuous base probably is an erosional contact. 2.3 ft (0.7 m) thick 

where measured north of mine. Relative thicknesses of units 6 and 7 changes somewhat 

laterally; within the mine area they are of nearly equal thickness.

Unit 6

Generally noncalcareous, moderately fissile, coarse- to medium-grained siltstone. 

Dark brownish red on both fresh and weathered surfaces. Parallel laminated; laminae are 

crudely planar to wavy. Weathers to thin plates 0.2-0.8 inch (0.5-2 cm) thick. Locally 

split into two nearly equal parts by a thin parting of fissile siltstone that wedges out 

laterally. 1.8 ft (0.6 m) thick.

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Unit 5

Porous, noncalcareous to locally slightly calcareous, weakly to moderately 

cemented, very fine grained to fine-grained quartz sandstone. Brick red on both fresh 

and weathered surfaces. Locally friable; weathers to highly rounded forms. Prominently 

crossbedded in lower fourth of bed; remainder massive to crudely parallel-bedded. 12 ft 

(3.7 m) thick.

Unit 4

Porous, slightly to moderately calcareous, weakly to moderately cemented, finegrained quartz sandstone. Slightly paler in color and better cemented than underlying 

unit. Appears massive from a distance but is internally crossbedded. 3 ft (0.9 m) thick.

Unit 3

Porous, slightly calcareous, weakly to moderately cemented, very fine grained 

quartz sandstone. Brick red on both fresh and weathered surfaces. Parallel bedded. 

Forms minor reentrant in canyon wall north of mine. 1 ft (0.3 m) thick.

Unit 2

Porous, weakly to moderately cemented, noncalcareous, very fine grained to finegrained sandstone. Weathers to rounded forms with pitted surfaces. Brick red to pale 

brick red on weathered surfaces; similar or slightly darker when freshly broken. 

Prominently crossbedded in sets 8-12 inches (20-30 cm) in height in lower fourth of bed, 

increasing to 3-6 ft (1-2 m) in remainder of bed. Locally becomes coarser grained, better 

cemented, and slightly calcareous toward top of unit. 26 ft (7.9 m) thick.

The adit at the northeast end of the mine (No. 3 adit of Miller, 1954 and fig. 17 of 

this paper) was driven into this sandstone.

Unit 1

Deep brownish-red, fissile, noncalcareous quartz-rich siltstone. Nonresistant; 

forms reentrant in canyon wall northeast of mine, adjacent to the north edge of the dump 

material below the mine road. Weathers to thin plates. Consists of two fining-upward 

units; the base of each is massive to platy and passes upward into progressively more 

fissile, finer-grained rock. Not exposed in surface workings of mine, which are entirely 

above stratigraphic position of this unit. 3 ft (0.9 m) thick.

75