White Mica Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine -- White Mica Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine By ROBERT A. AYUSO U.S. GEOLOGICAL SURVEY BULLETIN 1803 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE: 1987 For sale by the Books and Open-File Reports Section, U.S. Geological Survey Federal Center, Box 25425, Denver, CO 80225 library of Congress Cataloging in Publication Data Ayuso, Robert A. White mica geochemistry of the Catheart Mountain porphyry copper deposit, Maine. (U.S. Geological Survey bulletin ; 1803) Bibliography: p. Supt. of Docs. no.: I 19.3:1803 1. Mica-Maine-Somerset County. 2. Porphyry-Maine-Somerset County. 3. Geochemistry-Maine-Somerset County. I. Title. II. Title: Catheart Mountain porphyry copper deposit. Ill. Series. QE75.89 no. 1803 557.3 s 87~00317 [QE391.M6] [549' .67] CONTENTS Abstract 1 Introduction 1 Analytical procedures 1 Geology 2 Summary of the petrography 3 Summary of rock bulk composition 5 Definition of white mica 7 Composition of white micas at Catheart Mountain 7 Regular variations in white mica composition 9 Discussion 9 Relation of white micas and bulk composition 9 Aluminous white micas in mineralized rocks 11 Summary 14 Acknowledgments 14 References cited 14 FIGURES 1. Location map and surface expression of hydrothermal alteration of the Catheart Mountain porphyry Cu-Mo deposit 2 2-9. Graphs showing: 2. Variation in Al203 and Cu, the wide range in Fe203 and Cu, and the variation in F and Cu in the mineralized bulk rocks at Catheart Mountain 5 3. Wide range in S as a function of Cu, the variation in Mo and Cu, and the variation in Sn and Cu in the mineralized bulk rocks at Catheart Mountain 7 4. Change in Fe+ Mg+ Mn as a function of 2-Alv 1 in the white micas from Catheart Mountain 12 5. Distribution of Na/(Na + K) and Si in the white micas from mineral­ ized rocks and unmineralized rocks from Catheart Mountain 12 6. Distribution of Al1v + Alv 1 and Si+(Mg+ Fe) in the white micas from mineralized rocks and unmineralized rocks from Catheart Mountain 12 7. Distribution of Si and Al1v + (Na + K) in the white micas from miner­ alized rocks and unmineralized rocks from Catheart Mountain 12 8. Distribution of 2Alv 1 and Mg+Ti in the white micas from mineral­ ized rocks and unmineralized rocks from Catheart Mountain 13 9. Change in Cu content of the bulk rock and the Al1v + Alv 1 in the white micas coexisting with sulfides, and the distribution of S in the bulk rock and the Al1v + Alv 1 in the white micas coexisting with sulfides 13 Contents Ill TABLES 1. Summary of hydrothermal alteration, mineralogy, and sulfide occurrence in the Catheart Mountain Cu-Mo deposit, Maine 4 2. Representative analyses of the mineralized and unmineralized rocks in the Cat­ c-~a ~ !lllliH<tal l CCO'.:t~ 3. Average compositions, standard deviations, and structural formulas of white mica in the potassic alteration zone at Catheart Mountain 8 4. Average compositions, standard deviations, and structural formulas of white mica in the quartz sericite, sericitic, and propylitic alteration zones at Catheart Mountain 10 IV Contents White Mica Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine By Robert A. Ayuso Abstract and, thus, it is potentially a good monitor of the changing chemical environment. This paper discusses the compo­ White micas from hydrothermally altered and mineral­ sition of white mica from different hydrothermal alter­ ized zones in the Catheart Mountain Cu-Mo porphyry ation zones in the Catheart Mountain Cu-Mo deposit in deposit have regular compositional variations that are gen­ erally related to the contents of copper, total iron, and Maine (fig. 1) as a function of the copper content of the sulfur in the whole rock. Micas in unmineralized rocks whole rock. exhibit no such relationship. White mica compositions Recent investigations on the composition of white reflect primarily the control imposed by the celadonitic sub­ micas in porphyry copper deposits include studies by stitution AIIV +Aiv1=Si+ (Fe, Mg). Guilbert and Schafer (1978), Beaufort and Meunier The variation in white mica composition in the Cat­ (1983), and Parry and others (1984). Guilbert and Schafer heart Mountain porphyry may be explained by super­ (1978) and Guilbert (1980) suggested that white mica in position and overprinting of mineral assemblages by ore porphyry mineralized areas might be an aid in explora­ fluids during mineralization and during the waning stages tion. McDowell and Elders (1980) studied white micas of hydrothermal alteration. Local bulk compositional differ­ from a geothermal field, and Hendry (1981) used white ences, changing PHao and fluid compositions during miner­ alization, and superposition of mineral assemblages along mica composition to study wall-rock alteration in volcanic­ microzoned veins contributed to variation in white mica hosted copper mineralization. Gunow and others (1980) composition. However, the general relation between white studied the fluorine content of sericite in the Henderson mica composition and base-metal content in the mineral­ molybdenite deposit in Colorado. In addition, the possi­ ized rocks at Catheart Mountain suggests that identification bility that white mica composition changes as a function of of this white mica population might be useful as an indica­ proximity to mineralization has been investigated in hydro­ tor of mineralization gradients. thermally altered submarine exhalative ores (Plimer and de Carvalho, 1982), in stratiform sulfide deposits (McLeod and Stanton, 1984), and in stratabound Ba-Zn INTRODUCTION mineralization (Fortey and Beddoe-Stephens, 1982). Mineralized rocks from porphyry copper deposits represent a complex interaction of magmatic and hydro­ ANALYTICAL PROCEDURES thermal processes. Titley (1982) and Beane and Titley (1981) recently summarized the geologic complexity of Twenty-two samples from the Catheart Mountain most porphyry copper deposits and noted the variation of Cu-Mo deposit were obtained from representative drill the intensive parameters before, during, and after miner­ cores traversing the regional hydrothermally altered and alization. Thus, the observed mineral assemblage depends mineralized zones (fig. 1). Polished microprobe thin sec­ on composition of the host rock, on position of the rock tions were obtained as close as possible to single veins in with respect to the ore fluid and heat sources, and on the drill core splits analyzed for major and trace elements as intensity and efficiency of late hydrothermal alteration well as for base metals (Mo, Cu, Sn, W, and so on). An events. effort was made to obtain the thin-section samples from Mineralogic studies may be of great value in under­ areas in the drill core that clearly contained chalcopyrite standing the chemical environment during mineralization and molybdenite in close proximity to white mica. in porphyry copper systems. For example, white mica Electron microprobe analyses of white mica were occurs in the potassic, phyllic, and mineralized zones of done at the U.S. Geological Survey, Reston, Va. Initially, most porphyry copper systems (Creasey, 1959; Meyer and the micas were analyzed with an automated, three­ Remley, 1967; Lowell and Guilbert, 1970; Rose, 1970) channel electron microprobe employing a combination of Analytical Procedures chemical, emission-spectrographic, and instrumental neutron-activation analyses. A complete presentation of the bulk-rock analyses and a discussion of the bulk chemistry are in preparation. GEOLOGY The Catheart Mountain Cu-Mo deposit is in a shallowly emplaced, fine- to medium-grained pluton that intruded the southeast flank of the Boundary Mountain anticlinorium (Albee and Baudette, 1972); this pluton belongs to a group of mineralized intrusives within the Appalachian porphyry province (Hollister, 1978). The two most important rock types at Catheart Mountain are quartz monzonite and quartz-feldspar porphyry, both of which contain mineralization. Nearby mineralized plutons in northern Maine include the newly reported Mo miner­ [] Quartz-sericitic 500 1000 METERS alization at Priestly Lake (Ayuso and Shank, 1983; Ayuso, 1984) and the Cu-Mo mineralization at Deboullie (Boone, Propylitic rn 1962; Hollister, 1978). The Catheart Mountain quartz monzonite and Figure 1. Location map and surface expression of hydro­ quartz-feldspar porphyry occur within the Attean Quartz thermal alteration of the Catheart Mountain porphyry Cu­ Monzonite of Ordovician age; this occurrence has led to Mo deposit. Also shown are the drill-hole sites. the assumption that the Attean Quartz Monzonite and the rocks at Catheart Mountain are genetically related and may be of the same age. However, petrographic studies wave-length and energy-dispersive procedures. Approxi­ have indicated that rocks at Catheart Mountain are sig­ mately half of the analytical work was done using an nificantly different from those from the Attean Quartz 1 automated ARL-SEMQ nine-channel electron Monzonite, and that they constitute a distinct mappable microprobe under similar operational conditions as in the unit. first part of the study, including 15-kilovolt operating The Attean Quartz Monzonite is coarsely voltage, 0.1-microamp beam current, and data-acquisition porphyritic and intrudes volcanic and volcaniclastic rocks (McGee, 1983) and correction procedures by the Bence that unconformably overlie the Precambrian (Naylor