DESCRIPTIVE MODEL OF SOLUTION-COLLAPSE
BRECCIA PIPE URANIUM DEPOSITS
By Warren I. Finch
SYNONYM: Collapse breccia pipe deposits,
sedimentary breccia pipe deposits, Orphan Lode-type deposit.
DESCRIPTION: Uraninite and associated sulfide,
arsenide, sulfate, and arsenic-sulfosalt minerals as disseminated replacements
and minor fracture fillings in distinct bodies in near-vertical cylindrical
solution-collapse breccia pipes, 30-175 m in diameter and 1,000 m in length.
Pipes located in flat-lying upper Paleozoic and Triassic rocks restricted
to the Grand Canyon region in the southwestern part of the Colorado Plateau.
TYPICAL DEPOSITS: Orphan Lode (Chenoweth,
1986; Gornitz and
others, 1988), EZ-2 (Krewedl
and Carisey, 1986), Pigeon (Schafer,
1988) all in Arizona.
RELATIVE IMPORTANCE: One of two dominant
high-grade sources of United States uranium production in 1987; expected
to be major source of future uranium production within the United States.
OTHER COMMODITIES: ± Cu± V± Ag± Au
ASSOCIATED DEPOSIT TYPES (*suspected
to be genetically related): *Sandstone uranium; supergene enrichment
of Cu and V and depletion of U in deeply eroded and weathered pipes--typical
example, Ridenour, Arizona (Chenoweth,
1988); Apex germanium- and gallium-bearing breccia pipe nearby in
Basin and Range province (Wenrich
and others, 1987).
REGIONAL GEOLOGIC ATTRIBUTES
TECTONOSTRATIGRAPHIC SETTING: Pipes found
within and along the southwest margin of the Colorado Plateau, in a stable
block existent since the Precambrian and resistant to tectonic forces
acting on the western part of the North American plate. Wall rocks of
pipes were deposited on a stable marine platform. Pipes apparently originated
along and at intersections of N. 50° E.- and N. 45° W.-trending joint
or fracture sets (Wenrich
and Sutphin, 1989), roughly parallel to orthogonal Colorado River
(N. 45° E.), Zuni (N. 45° W), and related lineaments shown by Green
(1988, fig. 4) that developed in the Precambrian and rejuvenated in later
periods. No igneous rocks are found in the pipes.
REGIONAL DEPOSITIONAL ENVIRONMENT: Breccia
pipes developed from solution collapse within the thick Mississippian
Redwall Limestone (0-210 m) beginning in the Late Mississippian and propagated
upward into overlying strata of carbonate-cemented sandstone, siltstone,
limestone, and conglomerate for at least 1,000 m, apparently only where
the Redwall is >15 m thick. Stoping was intermittently active and reached
the lower members of the Chinle Formation in Late Triassic time.
AGE RANGE: Host wall-rocks for pipes: Late
Mississippian to Late Triassic. Ores: 260-200 Ma (Ludwig
and Simmons, 1988).
LOCAL GEOLOGIC ATTRIBUTES
HOST ROCKS: Karst-collapse breccia.
Breccia clasts as wide as 10 m across, consisting mainly of sandstone
(~90 percent) and siltstone (~10 percent), occur in a matrix of quartz
grains that is commonly well-cemented with carbonate minerals. Minor claystone
and limestone clasts.
ASSOCIATED ROCKS: Unbrecciated flat-lying
sandstone, siltstone, and limestone.
ORE MINERALOGY: Principal ore minerals:
uraninite±roscoelite+tyuyamunite* +torbernite*+uranophane *+zeunerite*+chalcopyrite+bornite*±chalcocite*±malachite*+azurite
*+brochantite *+volborthite+naumannite. Associated base-metal minerals:
±sphalerite ±galena±bravoite± rammelsbergite+stibnite +molybdenite+skutterudite.
An asterisk indicates sugergene origin. Pre-uraninite mineral assemblages
resemble those of Mississippi Valley-type deposits. Unusual complexity
of mineralogy shown in appendix E.
GANGUE MINERALS: Pyrite+marcasite+calcite+dolomite+barite+anhydrite±siderite
±hematite±limonite±goethite±pyrobitumen (see app. E).
AND MINERAL ZONING: Orebodies occur as discontinuous pods mainly in
the core of the breccia pipe but some are also found in the annular-ring
structure and may occupy as much as a 200-m vertical interval (fig. 20).
Mainly replacement and sparse open-space filling. Pyrite/marcasite and
base-metal sulfides, locally associated with pyrobitumen, form a discontinuous
"massive sulfide cap" above the uranium deposits in many pipes.
Uranium, vanadium, and copper roughly zoned within some deposits.
Figure 20. Schematic cross section of a
solution-collapse breccia pipe in the Grand Canyon region, showing the
general distribution of uranium ore within the pipe (stratigraphic section
modified after Van Gosen
and Wenrich, 1989). [Because of limitations on the reproduction of
the original figure, the figure shown here does not contain all of the
detail of the original.]
ORE CONTROLS: Fractured, permeable rock
within breccia pipe. Nearly all primary ore confined to the breccia pipe:
rarely, a little uranium ore is reported in relatively undisturbed beds
outside the ring structure. Vertically, most primary ore is below the
Coconino Sandstone and at the level of the Hermit Shale and the Esplanade
Sandstone of the Supai Group (fig. 20).
ISOTOPIC SIGNATURES: See Age Range above.
FLUID INCLUSIONS: Fluid-inclusion-filling
temperatures of 80-173°C for ore-related sphalerite, dolomite, and calcite.
Salinities (in weight percent NaCl equivalent) are for sphalerite, [=
or >] 9, for dolomite, [= or >] 17, and for calcite, [= or >]
4 (Wenrich, 1985;
Wenrich and Sutphin, 1988).
STRUCTURAL SETTING: All ore associated
with solution-collapse breccia pipes.
ORE DEPOSIT GEOMETRY: Orebodies develop
in annular-ring structures and in the core (fig. 20). At Orphan Lode,
orebodies in core range from 15 to 60 m in diameter and from 30 to 90
m high; annular-ring orebodies are 5-20 m wide, and a few tens of meters
high, and extend variably part way around ring circumference (Chenoweth,
ALTERATION: Characteristic bleaching by
reduction (some extends locally outward into wall rocks as much as 30
m); common carbonate recrystallization and calcification, local dolomitization
and kaolinization, some weak silicification. Calcified rock extends
outside boundary shears, completely surrounding the Orphan Lode pipe.
Malachite, azurite, goethite, and other secondary minerals on surface
outcrops of eroded pipes.
EFFECT OF WEATHERING: Leaching of U and
enrichment of Cu and V, particularly in those pipes deeply weathered.
"Massive sulfide cap" apparently prevented oxidation prior to
erosion and exposure.
EFFECT OF METAMORPHISM: Not applicable.
GEOCHEMICAL SIGNATURES: Enrichment of Ag,
As, Ba, Cd, Co, Cr, Cs, Cu, Hg, Mo, Ni, Pb, Sb, Se, Sr, U, V, Y, Zn, Zr,
and REE; indicator elements are Ag, As, Co, Cu, Ni, Pb, and Zn (Wenrich,
GEOPHYSICAL SIGNATURES: Electrical conductivity
and magnetic properties of the pipes are significantly greater than for
unbrecciated rocks; diagnostic differences in conductivity shown by scalar
audiomagnetotelluric (AMT) and E-field telluric profile data for one pipe
(Flanigan and others,
1986). Ground magnetometer surveys show subtle low magnetic values
over several pipes (Van
Gosen and Wenrich, 1989).
SPATIAL EXPLORATION GUIDES: Collapse features
recognized by concentrically inward-dipping beds, circular concave topography,
circular patches of brecciated and (or) bleached or iron-stained rock
(related to "massive sulfide cap") and differences in vegetation.
In well-exposed areas of the Marble Plateau, collapse breccia pipe densities
are 0.11 pipes per square kilometer. Marked tendency for pipes to occur
in clusters as small as 3 km2 in diameter. The presence of one pipe indicates
a high probability for other pipes nearby.
OTHER EXPLORATION GUIDES: For a new area
outside of the Grand Canyon region, a thick (>15 m) flat-lying, karst-forming
limestone overlain by a thick sequence of predominantly carbonate-cemented
sandstone and siltstone within a perpetually stable cratonic environment
and a post-pipe formation volcanic source for uranium. Preexisting Mississippi
Valley-type Cu-Co-Ni-Pb-Zn sulfide-rich ore may be required as a reductant
for uranium deposition.
OVERBURDEN: Favorable area on Coconino
Plateau (fig. 20): depths to mineralized portion of pipes are 150-600
m. Area exposed on Esplanade surface (fig. 20): depths are 0-120 m. Additional
cover by basalt, 0-100 m thick, around San Francisco and Mt. Floyd volcanic
fields. Quaternary and Tertiary sediments, 0-50 m thick, cover a few areas.
OTHER: Tectonic stability required for
preservation. "Massive sulfide cap" prevented and delayed oxidation
of some breccia pipe ores. Goethite possible pathfinder mineral for recognition
of concealed pipe.