Pertinent physical properties of the upper Keweenawan rocks can be measured or in{erred within a sufficiently narrow range to make the quantitative evaluation of various paleohydrologic models for the origin of the White Pine copper deposit feasible. The approach is illustrated -here by calculations for models that involve lateral migration of fluids through the subjacent Copper Harbor Conglomerate to the site of the deposit and stripping of copper from these solutions where they percolated upward through the Nonesuch Shale. The calculations reveal limitations to theories of origin that would not be evident from purely qualitative consideration; some of these limitations could be useful to exploration. For example, if the water was yielded by compaction of the Copper Harbor Conglomerate and contained 50 ppm Cu, there must have been significant convergence-of solution paths toward White Pine. Surface water entering the Copper Harbor Conglomerate on the north limb of the Lake Superior syncline is an adequate source if it could be shown that the point of entry was once significantly higher in altitude than the water table at White Pine; this model implies a major copper deposit at great depth north of the axis of the syncline. Ground water entering the Nonesuch Shale up dip from White Pine is not a possible source of mineralizing solutions. Symbols Used be = thickness of Copper Harbor Conglomerate (cm). b• = thickness of Nonesuch Shale (cm). h' = excess head (cm) at base of Nonesuch Shale. z•h '= difference ooe excess head at base of Nonesuch Shale between two specified localities (cm). z•h • divided by the distance (cm) between the two localities is the average hydraulic gradient between the localities. K• = hydraulic conductivity of Copper Harbor Conglomerate (cm/sec). K• = hydraulic conductivity of Nonesuch Shale (cm/ sec). qh = rate of lateral movement of fluid in the Copper Harbor Conglomerate (cm/sec). {/, = rate of movement of fluid upward across the base of the Nonesuch Shale (cm/sec). x Publication authorized by the Director, U.S. Geological Survey. the volume of water flowing past a vertical strip 1 cm wide and extending from top to bottom of the Copper Harbor Conglomerate in 1 second •(cc/sec). Where that formation is essentially horizontal, Vh = q• X the volume of water flowing upward past a strip 1 cm wide parallel to the base of the Nonesuch Shale between two specified localities in 1 second (cc/sec). It is the product of the average rate of upward movement of fluid (q0 and the distance between the localities.
The formations of central and east-central Vermont are exposed as a series of parallel belts that strike nearly north. Most of the rocks dip steeply, and many are overturned. With one possible exception, there seem to be no major repetitions within the sequence, and the order of formations from west to east appears to be the same as the order of their deposition. The formations are dominantly schist or phyllite, with varying proportions of arenaceous material. One thin formation, the Shaw Mountain, contains quartz conglomerate, calcareous tuff, and crinoidal limestone. The third-from-highest formation, the Waits River, is very thick and contains a large proportion of calcareous beds. The distance from the base of the lowest formation to the top of the highest, measured normal to bedding, is more than 100,opo feet; this large apparent thickness is believed to be not very much greater than the original thickness.The metasediments have been intruded by granitic dikes and plutons, mafic dikes, and small ultramafic plu,¥>;~-principal stages of deformation are distinguished. During the earlier stage the rocks were folded, and a schistosity was developed nearly parallel to bedding. Throughout the area the minor folds of this stage indicate a consistent upward movement of rocks on the east with respect to those on the west. The folds plunge at low to moderately steep angles, typically northward. Phenomena associated with the later stage of deformation decrease in intensity both eastward and westward from the belt underlain by the calcareous Waits River formation. At a distance from this formation, the rocks have prominent slip cleavage, and the earlier schistosity is folded. The minor folds plunge moderately to steeply northward on the western side of the area and more gently northward on the eastern. As the Waits River formation is approached, slip cleavage passes gradually into a schistosity that obliterates the earlier schistosity, and the intensity of later folding mcreases. In both the eastern and the western parts of the area the later minor folds indicate that the rocks of the Waits River formation have moved upward with respect to the formations on either side.The central part of the belt underlain by the Waits River formation is marked by a huge arch, 10-20 miles across, whose axis is more or less parallel to the belt and plunges gently northward. This is shown to be an arch, not in bedding, but in the later schistosity and in the axial planes of large isoclinal folds that were formed during the later stage of deformation. The axial planes of three of these large isoclinal folds can be correlated across the crest of the cleavage arch at Strafford Village.
The subsurface structure of the western Lake Superior region iJ3 analyzed by combining information on surface geology with fl,eromagnetic, gravity, and paleomagnetic data. Surface attitudes and the map pattern suggest that the upper Keweenawan sedimentary rocks (Bayfield and Oronto Groups) have the general form of a lens, thickening southeastward from a featheredge close to the Minnesota shore of Lake Superior.
DISCUSSIONS THE WHITE PINE COPPER DEPOSITSir: Reno Sales (3) has offered a remarkably perceptive discussion of the paper by Wright and myself (6) on the White Pine copper deposit. Ira B. Joralemon has added a brief postscript (2) more or less endorsing Sales' views and calling attention to additional data. My reply can hardly be expected to prove the origin of the deposit, but perhaps it will show how many problems one must overlook or oversimplify to be satisfied with the conclusion that the deposit was formed in one episode by hydrothermal solutions of magmatic origin emanating from the White Pine fault.The main thesis of our original paper was that the distribution of shale ore shows virtually complete independence of the structure and is related in almost all its aspects to the lithology and paleogeography of the host beds. From this we concluded (6, p. 713) that "at least three general theories of origin satisfy these conditions: (1) the copper was dropped into or precipitated within the original mud; (2) the copper, diffusing downward into the uppermost few inches of mud, replaced some primary constituent of the mud, perhaps iron sulfide or calcium carbonate; (3) the copper was introduced into the rocks after lithification and presumably deformation, and the loci of deposition were controlled to an extraordinary degree by the physical or chemical character of the host rocks." At least one additional possibility, in some respects more satisfactory than any of the others, should have been added to the list: (4) the copper was introduced into the shales from below after they were buried but before they were deformed or compacted to anything like their present degree; this might be called a "diagenetic" or "cementation" hypothesis. On the basis of some relationships described below that have become apparent since our paper was written, I would favor this fourth hypothesis over the other three if it were essential to choose among them at this time.The version of the third hypothesis that Sales espouses necessarily denies the existence of a significant copper concentration at the site of the present ore body before the deformation and faulting of the area, and in this respect differs fundamentally from the other three hypotheses. The heart of the argument, therefore, lies in this point. Sales takes issue with the arguments for a pretectonic copper concentration as follows: (a) all of our plan maps show "a close relationship areally between the [White Pine] fault fissure and the ore distribution ;" (b) the existence of copper in fissures and the pattern of distribution of copper in sandstone show, as we pointed out in our original article, that at least some of the copper was emplaced after the faulting, and a hypothe- 402
The Orange County copper district in east-central Vermont includes an ar«a about 20 mile* long and 5 miles wide, elongate north-south* It contains thre* important «ine», the Elizabeth, Ely, and Eureka** Union* The district yielded important amounts of copper befor« 1882, mostly from the Ely mine, but has since been only a small and intermittent producero Total production is estimated to be between 50 and 60 million pounds of copper* folds in their vicinity. It it suggested that th* Eureka-Ohion de~ posits nay be connected to a similar shoot at moderate depth, and thia possibility could readily be tested by drilling* Geologic factors that should be considered in connection with geophysical prospecting are discussed, and a number of favorable areas for geophysical prospecting are described.Detailed descriptions of individual mines «rd prospects, ex*» eluding the Elisabeth nine, uhlch has been described in an earlier report, follow the general discussion*
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