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

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Previous section Volume contents 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 copperÐprecious 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 (≈72Ð70 lites, and peralkaline granites, rhyolites, and trachytes. Ma) and middle Tertiary (33Ð23 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 variations in modal mineralogy abound. In this paper we use MOUNTAINS whole-rock major-element oxide analyses to define alkaline rocks as those igneous rocks that either (1) have weight per- Laramide and younger alkaline rock-related gold deposits in the Rocky Mountains are listed in table 1, and some cent Na2O+K2O>0.3718 (weight percent SiO2) Ð14.5; or (2) typical ore-related rock assemblages are plotted on total al- have mol Na2O + mol K2O > mol Al2O3. Criterion 1 is from Macdonald and Katsura’s (1964) alkalis versus silica plot for kali-silica (TAS) variation diagrams in figure 3. Many of separating alkaline from subalkaline basalts (fig. 2). Criteri- these assemblages include relatively primitive mafic alka- on 2 defines peralkaline rocks in the sense of Shand (1951). line rocks together with highly evolved or fractionated Criteria 1 and 2 are independent; that is, peralkaline rocks as rocks. This combination suggests that crustal level parking defined by criterion 2 need not satisfy criterion 1. Note that (perhaps at neutral buoyancy levels) and fractionation have silica saturation (the presence or absence of either modal or been important processes in the evolution of these suites. normative feldspathoids) is not a criterion for alkaline rocks Coeval calc-alkaline rocks are common at many Rocky as used here. Alkaline rocks range in composition from Mountain alkaline rock localities (fig. 3EÐH) and are pre- relatively primitive kimberlites, lamproites, lamprophyres, dominant at some of them. In many cases the calc-alkaline 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 RositaÐSilver Cliff 2 Department of Geological Sciences, University of Colorado, Boulder, CO 80309. volcanic centers, Colorado (fig. 3H). 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 copperÐprecious 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° 105°

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 Volcanic Idaho Springs Field La Sal Ophir-San Miguel- Cripple Creek Mountains Klondike Ridge Rosita

Henry San Juan Mountains Abajo Volcanic Field Mountains

Navajo Plata 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 Bostonites copper production. Deposits of this type in Mesozoic Alkaline accreted terranes are being actively mined in British 10 Columbia (McMillan, 1991; Schroeter and others, 1989). Epithermal gold deposits.—These are generally associated with syenites, trachytes, phonolites, and lampro-

O, IN WEIGHT PERCENT High-silica phyres, and they occur in a variety of settings including 2 5 rhyolite Calc-alkaline volcanic vent complexes, breccia pipes, hot-spring and O + K 2 Alkaline-subalkaline boundary geyser systems, bonanza veins, and replacements and dis- Na 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 80 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 Katsura’s (1964) boundary for separating alkaline and subalkaline include the Cripple Creek district (Loughlin and Ko- rocks, and compositional fields for alkaline rocks related to schmann, 1935), the Boulder County telluride camps 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 Calc-Alkaline Laccoliths A Laccolith E 0 0 30 40 50 60 70 80 30 40 50 60 70 80 15 15 CRIPPLE CREEK, LITTLE ROCKY MOUNTAINS, COLORADO MONTANA N=53 N=31

10 10

5 5 Lamprophyre, basalt Trachyte Syenite, monzonite, Phonolite quartz monzonite, Syenite, latite-phonolite B F granite 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15 GOOSE LAKE STOCK, JUDITH MOUNTAINS, O, IN WEIGHT PERCENT

2 COOKE CITY, MONTANA MONTANA N=6 N=51 O + K 2

Na 10 10

5 5 Trachyte, tinguaite Granite Syenite, monzonite Syenite, monzonite, etc. C G 0 0 30 40 50 60 70 80 30 40 50 60 70 80 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 High-silica rhyolites D H (crustal melts) 0 0 30 40 50 60 70 80 30 40 50 60 70 80 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 assemblages (table 2). The ore fluids for both epithermal and porphyry copperÐprecious metal systems were CO2- rich and relatively oxidized. Fluid inclusions in vein and OXIDIZED CO2-RICH ALKALINE MAGMA rock minerals are CO2-rich. Alteration assemblages, both 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 CO2-rich alkaline magma chamber. From gangue minerals commonly include carbonates and sul- Mutschler and Mooney (1993). fates, and negative δ34S 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 concentrations, and in ore-fluid pressure, temperature, and A variety of gold-bearing deposits are associated with composition (table 2), suggesting that they were deposited alkaline rocks; consequently various techniques may be use- from separate fluids. Cameron and Hattori (1987) pro- ful in prospecting for different types of deposits (Mutschler posed a scheme for the essentially simultaneous develop- and Mooney, 1993). Some useful indicators are as follows: 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 Ä ), 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 copperÐprecious 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, CO2-saturated alkaline mag- parentheses): K-metasomatism (whole-rock K2O>Na2O; ma chamber exsolves a CO2-rich, highly saline, metal- hydrothermal K-feldspar and (or) biotite), carbonatic bearing aqueous fluid, which then unmixes into two im- (whole-rock CO2>0.5 weight percent; hydrothermal carbon- miscible phases: (1) a high-salinity fluid into which Cu, ate minerals; CO2-rich fluid inclusions), redox (whole-rock Fe, Ag, and PGE are partitioned as Cl complexes, which δ34 Fe2O3>1.5 FeO; hydrothermal hematite), S evidence that 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 H2OÐCO2 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

GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 243

4. Low-sulfide “invisible,” or micron- (micrometer) Mountains, Ariz. as Colorado Plateau features. In a similar size Au deposits may be recognized by geochemical anom- fashion, we have excluded the middle Tertiary West Elk alies in Au (>10 ppb) and some of the following: Ag, As, Bi, Mountains, Colo., laccoliths from the Colorado Plateau. Ce, F, Hg, Sb, Se, Te, Tl, U, V, W, and high Ba and Sr in Data on the lithology, form, age, and associated miner- Ba:Sr:Rb ratios. Gold, commonly at levels in the parts-per- alization of the igneous rocks in the Colorado Plateau lacco- billion range, is the only universal geochemical guide to ore; lithic centers (fig. 1) are summarized in table 3. Total alkali- concentrations and dispersion halos of other “pathfinder” silica (TAS) plots for those laccolithic centers for which elements can vary widely from deposit to deposit, even with- whole-rock chemical analyses are available are shown in fig- in a single district (Mutschler and others, 1985). ure 5. Most of the igneous rocks fall into two groups: 1. Dominantly calc-alkaline intermediate (55Ð65 weight percent SiO2) rocks, here collectively termed diorite ALKALINE ROCKS AND porphyry. Different investigators have applied various MINERALIZATION IN THE names to these rocks, including diorite-monzonite porphy- COLORADO PLATEAU ry, diorite porphyry, granodiorite, granodiorite porphyry, microgabbro, microgranogabbro, monzonite porphyry, LACCOLITHIC CENTERS plagioclase-hornblende porphyry, porphyritic adamellite, quartz diorite porphyry, and quartz monzonite porphyry. Depending on how the borders of the Colorado Plateau Diorite porphyry forms concordant plutons, including lac- are defined, laccolithic centers on the plateau can be coliths and sills, and discordant stocks, dikes, and bys- enumerated differently. We have somewhat arbitrarily maliths. It constitutes more than 90 percent of the igneous considered that the Laramide-age La Plata, Ouray, and Rico rocks exposed in the major laccolithic centers of the laccolithic centers in Colorado are Rocky Mountain fea- Colorado Plateau (Hunt, 1956; Hunt and others, 1953) and tures, whereas we have included the Laramide age Sleeping adjacent areas. On TAS plots most of the diorite porphy- Ute Mountain (or “Ute Mountains”), Colo., and Carrizo ries fall within the compositional fields of the mid-Tertiary

244 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH

15 15 ABAJO MOUNTAINS, MOUNT HILLERS, UTAH HENRY MOUNTAINS, UTAH N=17 N=9

10 10

5 5 Diorite porphyry (stock samples Diorite porphyry are circled) A E 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15 CARRIZO MOUNTAINS, MOUNT HOLMES, UTAH HENRY MOUNTAINS, UTAH N=3 N=3

10 10

5 5

Diorite porphyry Diorite porphyry B F 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15

O, IN WEIGHT PERCENT MOUNT ELLEN, MOUNT ELLSWORTH, 2 HENRY MOUNTAINS, UTAH HENRY MOUNTAINS, UTAH N=23 N=5 O + K 2

Na 10 10

5 5 Diorite porphyry Basalt Diorite porphyry C G 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15 MOUNT PENNELL, NORTH MOUNTAIN, HENRY MOUNTAINS, UTAH LA SAL MOUNTAINS, UTAH N=42 N=32

10 10

5 Peralkaline granite 5 Syenite porphyry Peralkaline rhyolite, granite porphyry Cumulate syenite inclusion Syenite porphyry Diorite porphyry D Diorite porphyry H 0 0 30 40 50 60 70 80 30 40 50 60 70 80 SiO2, IN WEIGHT PERCENT Figure 5 (above and facing page). Total alkali-silica plots for Colorado Plateau laccolithic centers. Data from references listed in table 3. 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, number of samples.

GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 245

15 15 MIDDLE MOUNTAIN, EARLY INTERMEDIATE LA SAL MOUNTAINS, UTAH LAVAS, BRECCIAS, PLUTONS N=8 N=130

10 10

5 5

Syenite porphyry Diorite porphyry I 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15 SOUTH MOUNTAIN, ASH-FLOW TUFFS LA SAL MOUNTAINS, UTAH (EXCLUDING LAKE CITY) N=5 CALDERA ERUPTIONS 30-26 Ma

10 10 N=123 O, IN WEIGHT PERCENT

5 2 5 O + K Diorite porphyry 2 J Na 0 0 30 40 50 60 70 80 30 40 50 60 70 80

15 15 OPHIR-SANMIGUEL- LAVAS AND PLUTONS O, IN WEIGHT PERCENT

2 KLONDIKE RIDGE, CONCURRENT WITH ASH- COLORADO FLOW TUFFS

O + K N=17 30-26 Ma 2

Na 10 10 N=117

5 5

Diorite porphyry K 0 0 30 40 50 60 70 80 30 40 50 60 70 80 SiO2, IN WEIGHT PERCENT 15 UTE MOUNTAIN, COLORADO Figure 6. Total alkali-silica plots for middle Tertiary (35Ð26 Ma) N=15 volcanic rocks and plutons of the San Juan volcanic field, Colorado. Data from Mutschler and others (1981). Boundary between alka- 10 line 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, number of

5 Lamprophyre alkaline-subalkaline rock divider in figure 5A, E, G, and H Diorite porphyry reflect post-crystallization alkali metasomatism, including (stock samples albitization (Nelson and Davidson, 1993) and potassic al- L are circled) 0 teration (represented by secondary potassium feldspar and 30 40 50 60 70 80 (or) biotite). SiO2, IN WEIGHT PERCENT 2. Alkaline intermediate (55Ð65 weight percent SiO2) calc-alkaline volcanics and plutons of the San Juan volca- rocks, here collectively termed syenite porphyry. These nic field, Colorado, as shown in figure 6. Many of the di- rocks occur at the Mount Pennell center in the Henry orite porphyry analyses that plot significantly above the Mountains and at the North and Middle Mountain centers

246 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH in the La Sal Mountains, Utah, forming stocks, sills, irreg- breccia pipes (volcanic vents?) and coeval small peralka- ular plutons, and dikes, all of which are younger than the line rhyolite porphyry plutons and dikes (Ross, 1992). diorite porphyry plutons. In addition to these well known Several of these pipes exceed 500 m in maximum horizon- localities, Condie (1964) has described a small quartz- tal dimension. A second alteration and mineralization bearing, highly oxidized, syenite porphyry pluton on the event is represented by disseminated hematite, minor vein southwest flank of the Navajo Mountain dome, Utah. chalcopyrite and pyrite, and vug and cone sheet-fracture Both nepheline- and quartz-normative syenite porphyries filling by calcite and quartz in the pipes. At North Moun- are present in the La Sal and Henry Mountains. Although tain, the final intrusive phases are late-stage to postminer- most of the La Sal syenite porphyries are peralkaline, none alization peralkaline rhyolite porphyry and nosean trachyte of the analyzed Henry Mountain syenite porphyries are. porphyry dikes that crosscut most intrusive phases, brec- The syenite porphyries of the Henry and La Sal Mountains cia pipes, and pervasively altered areas (Ross, this vol- are chemically distinct from, and have no counterpart in, ume). Peralkaline granite also occurs as the youngest the voluminous coeval mid-Tertiary rocks of the San Juan intrusive phase in the Mount Pennell center of the Henry volcanic field of southwestern Colorado, as shown by Mountains, where Hunt (1988) suggests it preceded or ac- comparing the TAS plots of figure 5D, H, and I with fig- companied mineralization. ure 6. In fact, in both the San Juan, Colo. (Mutschler and others, 1987), and Marysvale, Utah (Mattox, 1992; Rowley and others, this volume), volcanic fields, alkaline magmatism is essentially restricted to bimodal (alkali basalt/tra- EXPLORATION POTENTIAL FOR chybasalt-rhyolite) suites, which began to erupt during the GOLD IN THE COLORADO PLATEAU onset of extensional faulting at ≈23 Ma. LACCOLITHIC CENTERS Nelson and Davidson (1993) have presented isotopic evidence that both the diorite porphyry and syenite por- Three features that are characteristic of productive al- phyry magmas of the Henry Mountains were derived from kaline rock-related gold deposits in the Rocky Mountains the same mantle source, but that they probably represent (tables 1, 2) are also present in the Colorado Plateau lacco- different degrees, or depths, of partial melting. According lithic centers: to Nelson and Davidson the parental magmas for the two suites reacted with, and assimilated, crustal rocks at differ- 1. The centers include highly fractionated alkaline ent parking levels. plutons such as the syenites of Mount Pennell in the Henry Pervasive hydrothermal alteration and local base- and Mountains, North Mountain in the La Sals, and Navajo precious-metal mineralization followed emplacement of Mountain. Evolved peralkaline granites and rhyolites are the syenite porphyry stocks at both the North Mountain also present at Mount Pennell and North Mountain. center in the La Sals and the Mount Pennell center in the 2. Pervasive hydrothermal alteration, including alkali Henry Mountains (Hunt, 1958; Hunt, 1988; Irwin, 1973; metasomatism and propylitic, carbonatic, redox, pyritic, Nelson and others, 1992). Propylitic (chlorite-calcite-epi- and silicic alteration, has been reported at most of the lac- dote-magnetite-hematite) alteration generally extends far- colithic centers. thest from the stocks. Other, generally more proximal, 3. Subeconomic occurrences of hypogene gold have pervasive alteration assemblages include hornfels skarn been recognized in the Abajo, Henry, and La Sal Moun- (aegirine-augite, riebeckite, glaucophane, hedenbergite, ac- tains, and significant historic gold-silver production took tinolite, biotite, magnetite, hematite, garnet), alkali metaso- place in the OphirÐSan Miguel centers. matism (albitization and subordinate potassium feldspathization), carbonatic alteration (calcite, siderite?), In the last decade, several major mining companies redox alteration (hematitization), and sulfidization (py- have evaluated the precious metal potential of the Colo- rite±base-metal sulfides). Narrow breccia zones and small rado Plateau laccolithic centers with geochemical surveys, fissure veins occur locally, commonly following sheeted and, in the La Sal Mountains, with drilling programs. Al- zones. Hypogene vein minerals include quartz, carbonates, though some geochemical anomalies and limited mineral- fluorite, Au-bearing pyrite, chalcopyrite, bornite, galena, ized drill intercepts have been found, no economic ore sphalerite, molybdenite, magnetite, and hematite. Similar deposits were discovered. However, relatively large areas but less intense alteration and mineralization occurred in of geologically suitable terrain remain untested, especially and adjacent to some diorite porphyry plutons at most of for low-sulfide, low-grade gold deposits. Inasmuch as the other Colorado Plateau laccolithic centers. (See table such “invisible” deposits are still being discovered in and 3.) adjacent to well-studied mining camps (see Tooker, 1990, At the North Mountain center in the La Sals the alter- for example), we conclude that a modest potential exists ation and mineralization caused by the syenite porphyry for finding economic alkaline rock-related gold deposits in were accompanied and also followed by formation of the the laccolithic centers of the Colorado Plateau.

GOLD DEPOSITS IN COLORADO PLATEAU LACCOLITHIC CENTERS 247

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