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 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