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Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine

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Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine Maine Geological S urvey Studies in Maine Geology: Volume 4 1989 Geochemistry of the Catheart Mountain Porphyry Copper Deposit, Maine Robert A. Ayuso U.S. Geological Survey Reston, Virginia 22092 ABSTRACT The Ordovician Catheart Mountain pluton is the best example ofa porphyry copper system in the New England Appalachians. It intrudes the Ordovician Attean quartz monzonite in the Proterozoic Chain Lakes massif. The pluton consists of equigranular granodiorite intruded by porphyritic dikes of granite and granodiorite which contain up to 4% sulfides (pyrite, chalcopyrite, and molybdenite). The equigranular host rocks and the porphyritic dikes are generally chemically similar, showing scattered compositional variations which reflect changes imposed on the pluton during crystallization and hydrothermal alteration. Most rocks are hydrated, containing high C02 and high S. The most intensely altered parts of the pluton are potassic (up to 6.9 wt % KiO) and highly mineralized, especially with Cu (up to 3900 ppm), but also with Mo (up to 320 ppm). The host granodiorite and porphyritic dikes are depleted in Ca, Na, Fe2+ and Sr as a function of increasing Cu; the sulfide-rich rocks are enriched in Rb and they have high Fe3+/Fe2+, K/Na, and Rb/Sr values. Abundances and ratios of the highly-charged cations (e.g. Zr, Hf, Ta, and Th) in the Catheart Mountain pluton resemble those in the Devonian granodiorites in the northern Maine plutonic belt. Abundances and ratios of the highly-charged cations in the Sally Mountain pluton, a nearby body also containing Cu and Mo mineralization, differ significantly from those in the Catheart Mountain pluton suggesting that the two plutons are not comagmatic. This is in agreement with lead isotopic compositions indicating that the Sally Mountain and Catheart Mountain plutons did not have the same source. Magma for the Catheart Mountain pluton was probably generated in the subcontinental lithosphere from a source consisting of continental basement (Grenville or possibly Chain Lakes massif) and mantle-derived rocks. The host equigranular granodiorite represents a water-undersaturated magma emplaced into the upper crust. Volatile- and chloride-rich fluids in the porphyritic dikes leached the wall rocks and produced chemical reactions leading to the decomposition of magmatic minerals, remobilization of the major and many of the trace elements, oxidation of iron, and precipitation of chalcopyrite and molybdenite. Fluids may have been magmatic at the start of the hydrothermal activity, but included a component of high-180 fluid originating from meteoric sources toward the waning stages of alteration. INTRODUCTION The Catheart Mountain pluton is in the northern Maine amples of rocks associated with porphyry-style copper plutonic belt (Fig. I), where it intrudes the Ordovician Attean mineralization in the northern Appalachians of New England, quartz monzonite in the Proterozoic Chain Lakes massif, an and thus it is part of the so-called nonhem Appalachian porphyry allochthonous block that rests on Middle Proterozoic Grenvillian province (Hollister et al., 1974). basement (Zen, 1983; St-Julien et al., 1983; Stewart et al., 1985). The purpose of this paper is to summarize the field relations The Catheart Mountain pluton is probably one of the best ex- and major- and trace-element chemistry of the Cathean Moun- 139 R. A. Ayuso 0 500 I 000 meters 45'-\ I I I I ~ Attean pluton ~ i I I Alteration zones I rrt:rml Potassic ViS::/J Ouartz-sericitic ~ Propylitic ~ 7 t.I Figure I. Map showing location of the Catheart Mountain and Sally Mountain plutons, the Chain Lakes massif, and structural features in Maine. Line A-B represents the southernmost extent of unexposed Grenville basement (Zen, 1983). The generalized map of the Cat heart Mountain pluton shows the hydrothennal alteration zones, and location of the drill holes (solid circles) and surface samples (crosses) used in this study (modified from Schmidt, 1974). tain pluton in order to better understand the genesis of the copper Determining the magmatic affinities of the Catheart Moun­ mineralization. Chemical infonnation on the Catheart Mountain tain pluton is significant for exploration and economic assess­ pluton is used to investigate changes in major- and trace element ment of granitic rocks because this pluton contains the best compositions as a function of increasing degree of mineraliza­ described porphyry copper deposit in New England, and also in tion and alteration. This is done in an effort to establish the light of current research aimed at synthesizing the regional relative elemental mobility produced by the hydrothennal metallogenesis of felsic intrusives in the northern Appalachians. events. Information on the immobile elements can then be used Geochemical studies emphasizing zoning of the chemical to compare the chemical affinities of the least altered parts of the components in hydrothennally altered porphyry copper systems Catheart Mountain pluton with granodioritic plutons from north­ have been reported by Creasey ( 1965), Theodore and Nash ern Maine, and to establish geochemical constraints on the (1973), Jambor (1974), Cox et al. (1975), Olade and Fletcher sources of the felsic magmas. ( 1975), Chaffee (l 976), Ford (I 978), and Schwartz ( 1982). 140 Catheart Mountain porphyry copper deposit These studies were generally done in the belief that more detailed Lyons et al. (l986) reported that muscovite from a quartz­ knowledge of the compositional variations produced during the molybdenite-white mica vein from the Catheart Mountain pluton fonnation of porphyry copper deposits may help in locating had ages of 441 ± 8 Ma by the K-Ar method and 447 ± 10 Ma poorly exposed porphyry copper deposits. by the Rb-Sr method. This mineralization age is in good agree­ Previous work by Schmidt ( 1974) established a comprehen­ ment with the zircon age of 443 ± 4 Ma obtained on the Attean sive framework for the study of rock alteration in the Catheart quartz monzonite (Lyons et al., 1986), and thus is compatible Mountain copper deposit. Schmidt ( 1974) described the with a comagmatic relation between the Catheart Mountain hydrothennal alteration zones and provided a detailed descrip­ pluton and the Attean quartz monzonite. tion of the sulfide mineralogy. Subsequent investigations by Hydrothermal alteration is pervasive in the Catheart Moun­ Atkinson ( 1977) provided additional details of the alteration tain pluton. The freshest parts of the granitic rocks consist of zones in the Catheart Mountain pluton. alkali feldspar and plagioclase, quartz, biotite, accessory Ayuso ( 1987) described the increasingly more aluminous minerals (sphene, zircon, and apatite), and secondary minerals compositions in white micas from the Catheart Mountain pluton (epidote, sericite, chlorite, sphene, and hematite). In most as a function of increasing Cu and Mo content of the bulk rocks. samples, secondary minerals are abundant, and quartz and white Ayuso (I 986a) and Ayuso et al. ( 1987, 1988) reported on the Pb mica are the predominant phases. Hydrothermal alkali feldspar and 0 isotopic geochemistry of the Catheart Mountain pluton in and biotite are probably found only in the areas containing the a study of granite rocks from northern Maine. The main con­ most intensive alteration. In contrast, chlorite, calcite, and clusion of those studies was that the Catheart Mountain pluton epidote are widely distributed throughout the hydrothermal and the Devonian granodiorite plutons from northern Maine zones. were isotopically similar, suggesting that they had a similar Phenocrysts ofquartz are probably the only textural features source. In the Catheart Mountain pluton, lead-isotope values are retained from the original igneous fabric in the Catheart Moun­ relatively unradio~enic, and oxygen-isotope values indicate al­ tain pluton. Quartz grains are generally subhedral to euhedral teration by high-1 0 fluids in the more intensely altered rocks. and contain numerous embayments and indentations. Plagioclase phenocrysts are rare; most plagioclase grains are found in the groundmass where they are largely and progressive­ FIELD AND PETROGRAPHIC RELATIONS ly replaced by white mica and epidote. Biotite may have been the most abundant original mafic igneous mineral, but very little In addition to the Catheart Mountain pluton, two important primary biotite remains, as it is replaced by chlorite and epidote. igneous bodies are exposed in the area, the Attean quartz mon­ Hornblende may also have been a primary mineral in the zonite and the Sally Mountain pluton. Both the Catheart Moun­ Catheart Mountain pluton, but it did not survive the hydrother­ tain and Sally Mountain plutons intrude the Attean quartz mal event. monzonite, which in turn intrudes the southeast flank of the Petrographic study of rock alteration of drill core samples Boundary Mountains anticlinorium (Albee and Baudette, 1972) allowed Schmidt ( 1974) to determine the regional hydrothermal in the Proterozoic Chain Lakes massif (Naylor et al., 1973). alteration zones, and to suggest, on the basis of the general The Chain Lakes massif consists for the most part of diamic­ features of the alteration system, that it strongly resembled the t ite, metasedimentary rocks (e.g. quartzite, feldspathic zonation found in many other porphyry copper systems accord­ metasandstone), and some mafic metavolcanic rocks (Boone et ing to the model of Lowell and Guilbert (1970). Two main al., 1970), which were regionally metamorphosed to sillimanite centers
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