Explained by Robert A. Ayuso
On the choice of deposit models
Porphyry-type mineralization includes several types of deposits that have in common extensive and pervasive regional alteration zones, bulk mineable features, and chalcopyrite contained in hydrothermally altered intrusions and adjacent wallrocks. Thus, the general porphyry copper mineralization model (17) of Cox (1986) and Singer and others (1986) includes a number of variants that include porphyry copper-molybdenum (model 21a), porphyry copper-gold (model 20c), and porphyry copper, skarn-related deposits (model 18a). In fact, it is well known that a continuum of geological attributes exists between these types of deposits (see also, for example, discussion by Hammarstrom and others, 1993).
Although porphyry-type deposits were exploration targets in the northeastern United States in the 1970s, no porphyry copper deposits were brought into production. Moreover, intense exploration for all types of granitoid-associated deposits identified no major deposits or even subeconomic prospects of molybdenum, gold, tin, or tungsten. Several subeconomic porphyry prospects, however, have been found in northern Maine, northern New Hampshire, and northern Vermont.
Examples of Known Deposits
One of the first porphyry copper-molybdenum prospects to be recognized, and one of the best studied, is at Catheart Mountain (Schmidt, 1974; Ayuso, 1987, 1989; Ayuso and Foley, 1993a, b; Schmidt and Ayuso, 1993; Foley and Ayuso, 1992; Hollister, 1978). Together with the nearby Sally Mountain porphyry molybdenum-copper prospect in northwestern Maine, these Ordovician to Silurian stocks are characterized by intense alteration and fracturing. Other important prospects in this tract include the Devonian porphyry-copper-molybdenum (and gold?) Deboullie Mountain (Ayuso and Loferski, 1993; Loferski and Ayuso, 1993) and porphry molybdenum-copper at Priestly Lake in northern Maine (Ayuso and Shank, 1983).
The potassic-altered Ordovician to Silurian Catheart Mountain prospect is the largest and best exposed porphyry copper-molybdenum deposit in New England, occupying a large area of intensely altered, red-yellow-brown rocks. Fine to medium grained biotite-bearing equigranular granodiorites were intruded by quartz porphyritic granodioritic dikes associated with copper and molybdenum mineralization at Catheart Mountain. Regional alteration zones resemble those typical of porphyry copper-molybdenum deposits in the western United States (Lowell and Guilbert, 1970; Gustafson and Hunt, 1975). From least to most altered, these zones include the propylitic, outer phyllic, phyllic, quartzose potassic, and potassic. Sulfides (chalcopyrite, pyrite, molybdenite, sphalerite, stannite, galena) constitute as much as 4 volume percent of the rocks, and are concentrated in quartz stockworks and are also disseminated in the host rocks. The highest amounts of Cu and Mo as well as the highest Cu/Mo ratio is found in rocks having potassic alteration. Extensive supergene ore metal enhancement at Catheart Mountain is absent. Intense metasomatic reactions controlled by the quartz porphry dikes enhanced the potassium contents and produced major changes in the major-, trace-element, and isotopic (Sr, O, and Pb) compositions of the hydrated and sulfur-rich host granodioritic rocks. Fluid inclusion studies have also shown that the fluids at Catheart Mountain were unusually rich in carbon dioxide, in contrast to fluid compositions in the majority of porphyry copper-molybdenum deposits in western United States.
The nearby Ordovician Sally Mountain porphyry molybdenum-copper (?) prospect is characterized by mineralization in highly fractured, altered, evolved, and silicified granitic quartz porphyry plugs. Sulfide minerals consist of chalcopyrite and molybdenite but the mineralization is clearly dominated by molybdenite. No reliable data for grade and tonnage are known for this prospect. Alteration zones are poorly developed (especially quartz-sericite alteration). The geochemical evolution, which was evaluated on the basis of isotopic compositions and incompatible element ratios and abundances, is opposite and distinct from that documented at the Catheart Mountain prospect. The total amount of sulfide minerals is low; thus, this also contrasts with the higher abundances at the Catheart Mountain prospect. Despite their proximity, it is unlikely that the Catheart Mountain and Sally Mountain prospects developed from the same mineralizing event.
A mafic hornblende-clinopyroxene-bearing syenitic unit (associated with shonkinite and gabbro) in the composite (hornblende-biotite-bearing granodiorite) Devonian Deboullie stock contains a small zone of disseminated and vein-filling pyrite and rare chalcopyrite and molybdenite (Loferski and Ayuso, 1993). Alteration and mineralization is associated with quartz porphyry dikes which intruded medium to coarse syenite. The syenitic half of the stock was extensively drilled in the 1970s. On the basis of drilling data (unreleased) and surface mapping, mineralization in the syenitic unit is thought to have many of the features of classic porphyry copper-molybdenum deposits. However, surface exposures lack well-developed, extensive alteration zones and intensely fractured areas containing abundant sulfide minerals.
Deposits associated with the intermontane belt of alkaline plutons in British Columbia, Allard stock (CO), and the Trans-Pecos alkaline province (TX) generally resemble the Deboullie stock; these alkaline plutons have copper, gold, silver, and platinum-group element mineralization and have been defined as the alkaline gabbro-syenite association (Zientek, 1993). Unlike this association, the alkaline Deboullie stock (mafic syenite) is not part of an extensive belt associated with an active margin. Instead, the stock contains a unique compositional range that distinguishes it from the predominant calc-alkaline Devonian granitic rocks in northern Maine, northern New Hampshire, and northern Vermont. Moreover, the Deboullie stock is associated with a continent-collision environment, and is not related to an active orogenic margin.
Field and geochemical information on porphyry copper-gold-molybdenum deposits in the northern Appalachians have been obtained for the most part from field work in remote areas characterized by poor exposures (glacial and vegetation cover), and from generally incomplete results obtained by drilling. In the northeastern United States, the continuum of porphyry deposits is expressed by a wide range of sulfide contents (Cu/Mo ratios) and sulfide minerals, and a broad spectrum of host rock compositions (mafic syenites, granodiorites, and granites).
The overall results of field studies, geochemical investigations, and drilling at Catheart Mountain show it to be generally similar to other calc-alkaline porphyry copper systems. The explored part of the Catheart Mountain prospect may in fact represent the lower part of an eroded porphyry copper deposit. This constitutes a negative feature against further exploration and development of this mineralized system and against the potential of this tract. No reliable geologic data exist to confidently estimate the size of the Sally and Deboullie prospects, but available field information is consistent with the conclusion that they resemble porphyry copper-molybdenum deposits.
The Catheart Mountain prospect is included in the generalized porphyry Cu model (17) of Singer and others (1986) and both the estimated tonnage (25 million metric tons) and copper grade (0.4 percent) are slightly above the 10th percentile of all deposits included in the model.
The tonnage of the Catheart Mountain prospect falls below the deposits included in the porphyry Cu-Mo model (21a) of Singer and others (1986), which is a subset of the generalized porphyry copper model. The size of the Sally Mountain and Deboullie prospects are unknown, but there is no evidence that they are larger than Catheart Mountain. Therefore, even though molybdenum grades are not available, and molybdenum is an important constituent in all the known deposits, the porphyry Cu grade-tonnage model (17) is thought to be a more appropriate choice than the porphyry Cu-Mo model (21a) for estimating metal endowment of undiscovered deposits in New England.
Skarns are characteristically associated with the contact metamorphism of carbonate-bearing wallrocks by intrusion of shallow plutonic rocks associated with porphyry copper deposits. However, there are no known copper (or gold, iron, etc.) skarns deposits or major occurrences of this kind associated with the intrusion of porphyry copper deposits in northern (or coastal) Maine, northern New Hampshire, nor Vermont. In addition, the level of erosion in the northeastern United States is likely to be too deep for skarns deposits to be an important prospective target for exploration.
On the delineation of permissive tracts
The terrane that contains calc-alkaline Ordovician to Devonian alkaline plutonic rocks of northern Maine, northern New Hampshire and northern Vermont, also includes the Catheart Mountain, Sally Mountain, and Deboullie prospects, and is considered permissive for porphyry copper deposits, especially the porphyry copper-molybdenum subtype. Stocks thought to be potential hosts for porphyry copper-molybdenum deposits are generally structurally controlled and are known to occur within tectonic zones that were part of active margins (continental or island-arc margins). The features mentioned above favor the Ordovician granitic suite because tectonic models for this suite indicate that they were produced in an active margin. However, an unfavorable feature in this tract is that stocks in this area were not shallowly emplaced. Moreover, the most abundant stocks in the northeastern United States are Acadian, and they are thought to have formed in a continental-collision environment. This feature argues against such Acadian granitic rocks as important hosts of typical porphyry-type mineralization.
On the numerical estimates made
The team used the consensus method to make resource estimates for porphyry copper deposits in the permissive tract. The bulk of the geologic information suggests that the majority of the stocks that could have been associated as hosts of porphyry copper-gold-molybdenum mineralization lack key petrologic and mineralization features, including appropriate alteration and fracture intensities, and shallow emplacement depths resembling those known from mined deposits elsewhere. For the 90th, 50th, and 10th percentiles, the team estimated 1, 1, and 2 or more porphyry copper deposits consistent with the grade and tonnage model of Singer and others (1986).
Ayuso, R.A., 1987, Mineralized rocks of the Catheart Mountain Cu-Mo porphyry, Maine: Aluminous white micas as indices of mineralization: U.S. Geological Survey Bulletin 1803, 16 p.
Ayuso, R.A., 1989, Geochemistry of the Catheart Mountain porphyry copper deposit, Maine, in Tucker, R.D., and Marvinney, R.G., eds., Studies in Maine geology, v. 4—Igneous and metamorphic geology: Maine Geological Survey, p. 139-162.
Ayuso, R.A., and Foley, N.K., 1993a, Rb-Sr isotopic redistribution and character of hydrothermal fluids in the Catheart Mountain Cu-Mo deposit, Maine, in Scott, R.W., Jr., Detra, P.S., and Berger,
B.R., eds., Advances related to United States and international mineral resources—Developing frameworks and exploration technologies: U.S. Geological Survey Bulletin 2039, p. E45-E57.
Ayuso, R.A., and Foley, N.K., 1993b, Porphyry Cu and Cu-Mo mineralization in the northern U.S. Appalachian Mountains, in Fenoll Hach Alli, P., Torres-Ruiz, J., and Gervilla, F., eds., Current research in geology applied to ore deposits: Proceedings of the second biennial SGA (Society for Geology Applied to Mineral Deposits) meeting, Granada, Spain, 9–11 Sep 93, p. 601-604.
Ayuso, R.A., and Loferski, P.J., 1993, Trace element geochemistry of syenite and granodiorite in the Deboullie pluton, northern Maine: Geological Association of Canada, Program and Abstracts, v. 17, p. A5.
Ayuso, R.A., and Shank, S.G., 1983, Quartz-molybdenite veins in the Priestly Lake granodiorite, north central Maine: U.S. Geological Survey Open-File Report 83-800, 12 p.
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 76.
Foley, N.K., and Ayuso, R.A., 1993, Character of hydrothermal fluids in the Catheart Mountain Cu-Mo deposit, Maine—Constraints on modeling a mineralizing system: Geological Association of Canada, Program and Abstracts, v. 17, p. A34.
Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposits at El Salvador, Chile: Economic Geology, v. 70, no. 5, p. 857-912.
Hammarstrom, J.M., Zientek, M.L., Elliott, J.E., Van Gosen, B.S., Carlson, R.R., Lee, G.K., and Kulik, D.M., 1993, Mineral resource assessment for locatable minerals (exclusive of the Stillwater Complex), in Hammarstrom, J.M., Zientek, M.L., and Elliott, J.E., eds., Mineral resource assessment of the Absaroka-Beartooth study area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-207, p. G1-G74.
Hollister, V., 1978, Geology of the porphyry copper deposits of the western hemisphere: Society of Mining Engineers, American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, 219 p.
Loferski, P.J., and Ayuso, R.A., 1993, Mineral chemistry of biotite and apatite from syenitic rocks in the Deboullie pluton, northern Maine: Geological Association of Canada, Program and Abstracts, v. 17, p. A67.
Schmidt, R.G., 1974, Preliminary study of rocks alteration in the Catheart Mountain molybdenum-copper deposit, Maine: U.S. Geological Survey Journal of Research, v. 2, p. 189-194.
Schmidt, R.G., and Ayuso, R.A., 1993, Porphyry Cu and Cu-Mo mineralization in New England—Examples from Catheart Mountain and Deboullie plutons and implications for future exploration in the northern Appalachian Mountains, in Applications of recent geological concepts to exploration in the northern Appalachians, Third annual CIM geological field conference: Exploration and Mining Geology, v. 2, no. 4, p. 411–412.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu-Mo, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 116-119.
U.S. Bureau of Mines, 1987, An appraisal of minerals availability for 34 commodities: U.S. Bureau of Mines Bulletin 692, 300 p.
Zientek, M.L., 1993, Alkaline gabbro-syenite association, in Hammarstrom, J.M., Zientek, M.L., and Elliott, J.E., eds., Mineral resource assessment of the Absaroka-Beartooth study area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-207, p. G75-G78.