Potential for Alkaline Igneous Rock-Related Gold Deposits in the Colorado Plateau Laccolithic

20 Pages·1998·0.6 MB·English

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Potential for Alkaline Igneous Rock-Related Gold Deposits in
the Colorado Plateau Laccolithic Centers
By Felix E. Mutschler,1 Edwin E. Larson,2 and Michael L. Ross3
CONTENTS
Abstract.......................................................................................................................... 233
Introduction.................................................................................................................... 234
Acknowledgments.......................................................................................................... 234
Alkaline Rock-related Gold deposits of the Rocky Mountains...................................... 234
Prospecting Guides......................................................................................................... 241
Alkaline Rocks and Mineralization in the Colorado Plateau Laccolithic Centers......... 243
Exploration Potential for Gold in the Colorado Plateau Laccolithic Centers................ 246
References Cited............................................................................................................ 247
FIGURES
1. Index map of Rocky Mountain and Colorado Plateau localities...............................238
2. Total alkali-silica diagram showing igneous rock classification...............................239
3. Total alkali-silica diagrams for alkaline rock-related gold deposits,
Rocky Mountains.......................................................................................................240
4. Diagram showing ore fluid evolution in an alkaline magma chamber......................241
5. Total alkali-silica diagrams for Colorado Plateau laccolithic centers ......................244
6. Total alkali-silica diagrams for San Juan volcanic field............................................245
TABLES
1. Laramide and younger alkaline rock-related gold deposits in the
Rocky Mountains.......................................................................................................235
2. Attributes of alkaline rock-related precious metal systems.......................................239
3. Colorado Plateau laccolithic centers..........................................................................242
ABSTRACT or platinum-group elements, through transitional types, to
epithermal precious-metal-only deposits commonly charac-
Several types of productive gold deposits in the Rocky terized by Au>Ag. The alkaline rocks associated with these
Mountains, ranging in age from 79 Ma to 26 Ma, show a deposits represent mantle melts which fractionated in crust-
close spatial, temporal, and genetic association with alkaline al-level magma chambers. Coeval calc-alkaline igneous
igneous rocks. Deposit types range from porphyry rocks formed by crustal melting and magma mixing occur
copper–precious metal systems characterized by Cu>Ag>Au with the alkaline rocks at many localities.
233
234 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH
The igneous rocks of the Colorado Plateau laccolithic and alkali basalts to highly evolved felsic syenites, phono-
centers fall into two age groups: early Laramide (» 72–70 lites, and peralkaline granites, rhyolites, and trachytes.
Ma) and middle Tertiary (33–23 Ma). Calc-alkaline diorite The worldwide association of a variety of types of gold
porphyries are the most voluminous igneous rocks in these deposits with alkaline igneous rocks (Mutschler and Moon-
centers. Essentially coeval alkaline syenite porphyries occur ey, 1993) suggests a genetic relationship. Various possibil-
at Mount Pennell in the Henry Mountains, the North and ities have been suggested to explain the relationship:
Middle Mountain centers in the La Sal Mountains, and the 1. Parental alkaline magmas may be generated by
Navajo Mountain center. Small volumes of late-stage peral- partial mantle melting at sites where deeply penetrating fault
kaline granite and rhyolite are also present at the Mount Pen- systems extend through the crust (Cameron, 1990).
nell and North Mountain centers. The rock chemistry and 2. Gold may be transported from the deep mantle by
alteration-mineralization assemblages of the Colorado Pla- mafic alkaline magmas (Rock and others, 1989).
teau laccolithic centers were compared to those of productive
3. The generally high volatile content of alkaline mag-
Rocky Mountain alkaline rock-related gold deposits. This
mas (Bailey and Hampton, 1990; Webster and others, 1992)
comparison suggests a modest potential for discovery of gold
could provide ligands for gold acquisition, transport, and
deposits at several Colorado Plateau localities.
deposition (Cameron and Hattori, 1987; Mutschler and
Mooney, 1993).
INTRODUCTION
ACKNOWLEDGMENTS
A significant part of the gold production and reserves
from Laramide and younger ore deposits in the Rocky Many colleagues in academia, industry, and govern-
Mountains comes from hypogene deposits associated with ment have helped us to compile data on the alkaline igneous
alkaline igneous rocks (table 1, fig. 1; Mutschler and others, rocks of the Cordillera and their associated mineral deposits.
1990). In this report we compare the major-element chemis- For providing us with unpublished material we especially
try of these productive alkaline rock suites with chemical thank James E. Elliott, Fess Foster, Bruce A. Geller, Stephen
data from the igneous rocks exposed in the laccolithic R. Mattox, Thomas C. Mooney, and Peter D. Rowley. Con-
centers of the Colorado Plateau. The comparison suggests a structive reviews by Thomas Frost and Steve Ludington
possibility for discovery of alkaline rock-related gold helped to clarify both our ideas and our expression.
deposits at several Colorado Plateau laccolithic centers,
including the Henry and La Sal Mountains and Navajo
Mountain, all in Utah. ALKALINE ROCK-RELATED GOLD
Alkaline igneous rocks have been defined in many
DEPOSITS OF THE ROCKY
ways, and confusing nomenclature schemes based largely on
MOUNTAINS
variations in modal mineralogy abound. In this paper we use
whole-rock major-element oxide analyses to define alkaline
Laramide and younger alkaline rock-related gold de-
rocks as those igneous rocks that either (1) have weight per-
posits in the Rocky Mountains are listed in table 1, and some
cent NaO+KO>0.3718 (weight percent SiO) –14.5; or (2)
2 2 2
typical ore-related rock assemblages are plotted on total al-
have mol NaO + mol KO > mol AlO. Criterion 1 is from
2 2 2 3
kali-silica (TAS) variation diagrams in figure 3. Many of
Macdonald and Katsura’s (1964) alkalis versus silica plot for
these assemblages include relatively primitive mafic alka-
separating alkaline from subalkaline basalts (fig. 2). Criteri-
line rocks together with highly evolved or fractionated
on 2 defines peralkaline rocks in the sense of Shand (1951).
rocks. This combination suggests that crustal level parking
Criteria 1 and 2 are independent; that is, peralkaline rocks as
(perhaps at neutral buoyancy levels) and fractionation have
defined by criterion 2 need not satisfy criterion 1. Note that
been important processes in the evolution of these suites.
silica saturation (the presence or absence of either modal or
Coeval calc-alkaline rocks are common at many Rocky
normative feldspathoids) is not a criterion for alkaline rocks
Mountain alkaline rock localities (fig. 3E–H) and are pre-
as used here. Alkaline rocks range in composition from
dominant at some of them. In many cases the calc-alkaline
relatively primitive kimberlites, lamproites, lamprophyres,
magmas probably resulted from partial crustal melting by
heat and volatiles from mantle-derived alkaline magmas that
_________________________________________
either ponded in or underplated the crust. In these situations
1 Petrophysics Crisis Center, Department of Geology, Eastern mixing of calc-alkaline and alkaline magmas can produce a
Washington University, Cheney, WA 99004. variety of hybrid magmas as at the Rosita–Silver Cliff
2 Department of Geological Sciences, University of Colorado,
volcanic centers, Colorado (fig. 3H).
Boulder, CO 80309.
3 Utah Geological Survey, 2363 South Foothill Drive, Salt Lake City, Precious metal-bearing deposits associated with Rocky
UT 84109. Mountain alkaline igneous centers can be divided into three
GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 235
236 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH
GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 237
types, or deposit models. Attributes of the two end-member pegmatite dikes and segregations, endoskarns, exoskarns,
models are summarized in table 2. The third, transitional, and local immiscible sulfide concentrations; by relatively
model can show features of both end-member models. high sulfur abundance; and by Cu>Ag>Au or PGE (plati-
Porphyry copper–precious metal deposits.—These num-group elements). Examples include the Allard stock,
occur in or adjacent to shoshonitic syenite stocks and are La Plata Mountains, Colo. (Werle and others, 1984); the
characterized by precious metals contained in copper Goose Lake stock, Cooke City, Mont. (Elliott, 1972, 1974;
sulfides occurring in stockworks, disseminations, veins, Lovering, 1930); and the Cerrillos district, New Mexico
238 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH
115° 110° 110055°°
MONTANA
Little Rocky Mountains
NORTH
Moccasin Mountains
DAKOTA
Judith Mountains
Little Belt Mountains
Whitehall
SOUTH
45° Goose Lake DAKOTA
Northern
IDAHO
Black Hills
Caribou
Mountain
WYOMING
NEBRASKA
COLORADO
UTAH
40° Boulder County
Central City-
NEVADA Marysvale Idaho Springs
Volcanic
Field
(cid:1)(cid:0)La Sal Ophir-San Miguel- Cripple Creek
Mountains Klondike Ridge
Rosita
Henry San Juan
Mountains Abajo Volcanic Field
Mountains
Navajo
(cid:1)(cid:3)(cid:0)(cid:2)(cid:1)(cid:3)Plata(cid:0)(cid:2) Mountains
Mountain Ute
Carrizo
Mountains
Mountains
Cerrillos
35° Ortiz
San Pedro
Jicarilla
Mountains
Nogal
White Oaks
ARIZONA
NEW MEXICO
0 100 200 300 KILOMETERS
Figure 1. Rocky Mountain and Colorado Plateau localities discussed in text. Circles are Colorado
Plateau laccolithic centers; triangles are Laramide and younger alkaline igneous rock-related gold
deposits.
GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 239
(Giles, 1991). In alkaline rock porphyry copper deposits,
15
the precious metals constitute byproducts or coproducts of
T
N
E Bostonites copper production. Deposits of this type in Mesozoic
C
ER Alkaline accreted terranes are being actively mined in British
P
HT 10 Columbia (McMillan, 1991; Schroeter and others, 1989).
G
EI Epithermal gold deposits.—These are generally asso-
W
N ciated with syenites, trachytes, phonolites, and lampro-
O, I High-silica phyres, and they occur in a variety of settings including
2 5
+ K Calc-alkaline rhyolite volcanic vent complexes, breccia pipes, hot-spring and
O geyser systems, bonanza veins, and replacements and dis-
a2 Alkaline-subalkaline boundary
N of MacDonald and Katsura seminations in sedimentary and igneous rocks. They are
0 characterized by Au (Ag)-telluride and native gold miner-
30 40 50 60 70 8 0
SiO2, IN WEIGHT PERCENT alization, relatively low sulfur abundance, and commonly
by Au>Ag. Examples of bonanza epithermal vein deposits
Figure 2. Total alkali-silica plot showing Macdonald and
include the Cripple Creek district (Loughlin and Ko-
Katsura’s (1964) boundary for separating alkaline and subalkaline
schmann, 1935), the Boulder County telluride camps
rocks, and compositional fields for alkaline rocks related to
precious metal deposits and selected other rock types. Arrows (Saunders, 1991), and the Bessie G mine in the La Plata
show generalized fractionation trends. Mountains (Saunders and May, 1986), all in Colorado.
240 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH
15 15
LA PLATA MOUNTAINS, ORTIZ MOUNTAINS,
COLORADO NEW MEXICO
N=59 N=30
10 10
5 5
Lamprophyre Alkaline-hypabyssal
Pegmatite, Trachyte plutons
Stock Alkaline stock
A Laccolith E Calc-Alkaline Laccoliths
0 0
30 40 50 60 70 8 0 30 40 50 60 70 8 0
15 15
CRIPPLE CREEK, LITTLE ROCKY MOUNTAINS,
COLORADO MONTANA
N=53 N=31
10 10
T 5 5
N Lamprophyre, basalt
CE Trachyte Syenite, monzonite,
ER Phonolite quartz monzonite,
T P B Syenite, latite-phonolite F granite
H
G 0 0
WEI 30 40 50 60 70 8 0 30 40 50 60 70 8 0
N
O, I15 GOOSE LAKE STOCK, 15 JUDITH MOUNTAINS,
2 COOKE CITY, MONTANA MONTANA
K
+ N=6 N=51
O
2
a
N10 10
5 5
Trachyte, tinguaite
Granite
Syenite, monzonite Syenite, monzonite, etc.
C G
0 0
30 40 50 60 70 8 0 30 40 50 60 70 8 0
15 15
BOULDER COUNTY TELLURIDE ROSITA-SILVER CLIFF,
CAMPS, COLORADO COLORADO
N=39 N=40
10 10
Lamprophyre
5 5 Alkaline plutons
(mantle melts)
Transitional rocks
Lamprophyre (mixed magmas)
Quartz syenite, bostonite
D H High-silica rhyolites
(crustal melts)
0 0
30 40 50 60 70 8 0 30 40 50 60 70 8 0
SiO2, IN WEIGHT PERCENT
Figure 3. Total alkali-silica plots for selected alkaline rock suites related to gold deposits in the Rocky Mountains. Data from references
listed in table 1. Boundary between alkaline and calc-alkaline rocks shown by inclined line. Curved arrows show generalized fractionation
trends: upper arrow for ore-related alkaline rocks; lower arrow for calc-alkaline rocks. N indicates number of samples.
GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 241
Noteworthy low-grade bulk-tonnage epithermal deposits
EPITHERMAL
include those of the Little Rocky (Russell, 1991), Mocca- DEPOSITS
sin (Kurisoo, 1991), and Judith (Giles, 1983) Mountains, PORPHYRY
DEPOSITS
Mont.
METEORIC WATER
Transitional epithermal-mesothermal gold-silver
deposits.—These include disseminated, breccia-pipe, skarn
and vein Au-Ag (Cu, Pb, Zn, W) deposits related to syen- CO2-RICH SALINE AQUEOUS
LOW SALINITY LIQUID
ite-monzonite-diorite plutons. The disseminated deposits
LIQUID
range from Au-only porphyry to sedimentary-rock-hosted
micron-size Au. Gold mineralization in the porphyry and S-, Te-COMPLEXES CI-COMPLEXES
Au, Sb, As Fe, Cu, Ag (Pb, Zn)
skarn deposits typically appears to be late in the paragenet-
ic sequence (post-base metal) and to be accompanied by
retrograde alteration events. Examples include the Ortiz IMMISCIBLE PHASES
Mountains (Maynard and others, 1991), Jicarilla Mountains
(Allen and Foord, 1991), and perhaps the White Oaks
district (Ronkos, 1991), New Mexico; the Red Mountain UNMIXING
area, Judith Mountains, Mont. (Hall, 1976); and some of
the Tertiary districts in the northern Black Hills, S.Dak.
AQUEOUS PHASE
(Paterson and others, 1988).
H2O-CO2-Cl-S-Metals
These three deposit models may represent a vertical FLUID
(and perhaps short-term temporal) progression. All three
types are associated with chemically similar alkaline rocks
SILICATE PHASE
(see fig. 3) and show similar hydrothermal alteration as-
semblages (table 2). The ore fluids for both epithermal
and porphyry copper–precious metal systems were CO -
2
rich and relatively oxidized. Fluid inclusions in vein and OXIDIZED CO2-RICH
ALKALINE MAGMA
rock minerals are CO -rich. Alteration assemblages, both
2
pervasive ones and those found as envelopes around veins, Figure 4. Schematic model for the evolution of two ore fluids
feature carbonate minerals and hematite. Ore-stage from an oxidized CO -rich alkaline magma chamber. From
2
gangue minerals commonly include carbonates and sul- Mutschler and Mooney (1993).
fates, and negative d 34S values in sulfides are common.
The two end-member deposit types differ, however, in Au, PROSPECTING GUIDES
Ag, PGE, Cu, and S abundances, in volatile-element con-
A variety of gold-bearing deposits are associated with
centrations, and in ore-fluid pressure, temperature, and
alkaline rocks; consequently various techniques may be use-
composition (table 2), suggesting that they were deposited
ful in prospecting for different types of deposits (Mutschler
from separate fluids. Cameron and Hattori (1987) pro-
and Mooney, 1993). Some useful indicators are as follows:
posed a scheme for the essentially simultaneous develop-
1. The source-host alkaline rocks for both porphyry
ment of two chemically distinct fluids in an oxidized (high
and epithermal mineralization show evidence of significant
ƒ ), CO -rich magma chamber, which Mutschler and
O2 2 crustal-level fractionation; thus chemically diverse suites of
Mooney (1993) modified to explain the formation of epith-
alkaline (eralization.
ermal “gold-only” deposits above and (or) peripheral to al-
2. Both porphyry and epithermal deposits are accom-
kaline rock-related porphyry copper–precious metal
panied by one or more of the following pervasive hydrother-
systems. This model is shown diagrammatically in figure
mal alteration assemblages (diagnostic characteristics in
4. A fractionated, oxidized, CO -saturated alkaline mag-
2 parentheses): K-metasomatism (whole-rock K O>Na O;
2 2
ma chamber exsolves a CO -rich, highly saline, metal-
2 hydrothermal K-feldspar and (or) biotite), carbonatic
bearing aqueous fluid, which then unmixes into two im-
(whole-rock CO >0.5 weight percent; hydrothermal carbon-
2
miscible phases: (1) a high-salinity fluid into which Cu, ate minerals; CO -rich fluid inclusions), redox (whole-rock
2
Fe, Ag, and PGE are partitioned as Cl complexes, which Fe O >1.5 FeO; hydrothermal hematite), d 34S evidence that
2 3
can form porphyry copper mineralization with high Ag sulfide S equilibrated with sulfate S, or sulfidization (hydro-
and (or) PGE values; and (2) a low-salinity H2O–CO2 flu- thermal pyritization and (or) sulfate minerals).
id into which Au (±As, Hg, Sb) is partitioned as S and (or) 3. Concealed porphyry and skarn deposits, which have
Te complexes which can form epithermal Au-dominated relatively high sulfide concentrations, may be recognized by
mineralization. induced polarization surveys.
242 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH