THE CONWAY OF NEW HAM1PSHIRE AS A MlAJOR LOW-GRADE RESOURCE BY J. A. S. ADAMS, M.-C. KLINE, K. A. RICHARDSON, AND J. J. W. ROGERS

DEPARTMENT OF GEOLOGY, WILLIAM MARSH RICE UNIVERSITY, HOUSTON, TEXAS Communicated by M. King Hubbert, September 21, 1962 Before the present work was undertaken, the reserves of thorium at all ore grades in the United States were much less than those of . All recent studies agreed on this point, and a report of the U. S. Atomic Energy Commission1 con- cluded: "In summary, both reserves and potential annual production of uranium must be considered to be several times those of thorium at current prices, but at some price, presumably many times the present price, thorium availability might approach that of uranium." McKelvey,2 among others, accepted the conclusion that this difference in reserves was also an indication of relative availability. The relative availability of thorium and uranium is of current importance in considering what types of nuclear-energy systems to investigate and develop. In reactors oper- ating in the thermal- and intermediate-neutron energy range, thorium-based fuel cycles have the advantage of higher conversion ratios, longer reactivity lifetimes, and lower fuel costs than are possible with natural uranium-based fuel cycles. Thorium-fueled reactors also have long-term potential for thermal breeding. Numerous hypotheses and arguments can be advanced to support the conclusion that the lower relative availability of thorium is real; there are also some indications that it may be only apparent. To resolve this question of relative thorium avail- ability, more data are needed beyond those already available.3 The present work was undertaken under Subcontract 1491 with the Oak Ridge National Laboratory to determine how much of this necessary information could be obtained with a mod- est expenditure. Relatively high concentrations of radioactive elements in the Conway granite and other members of the White Mountain series have been known for some time.4 Billings and Keevil5 found that radioactivity increases toward the younger members of the series and considered this increase to be caused by concentration of allanite, especially in the Conway granite. Whitfield6 reported the first quantitative tho- rium determinations in the Conway and various other granitic bodies. Butler7 reported an average thorium concentration of nearly 50 parts per million in nine samples of Conway granite. Lyons8 reported some thorium determinations on New Hampshire plutonic rocks, including the White Mountain magma series. The Conway Granite and lWhite Mountain Magma Series. Rocks of the White Mountain magma series occur largely in the White Mountains and in smaller outlying areas in New Hampshire (Fig. 1). The series consists of a sequence of plutonic rocks ranging from gabbros to granite and the locally distributed Moat Volcanics. The intrusive rocks occur in subjacent stocks or batholiths or as ring dikes. The Conway granite, the major member of the White Mountain series, is the youngest plutonic member and forms batholithic bodies transecting earlier structures and rock types. The second most abundant rock type is the Mount Osceola granite, which is distinguished from the Conway with difficulty in the field. 1898 Downloaded by guest on October 2, 2021 VOL. 48, 1962 GEOLOGY: ADAMS ET AL. 1899

Other members of the series are of 73°00 72°30' 72°00 71°30 71,00 smaller areal extent and have gener- ally not been included in the present study. The geology and petrology 4500' N of the White Mountain magma * * * 1 ~~~~~~~~~~~~~~~~~WhiteMdountawn Magmo Series p series have been extensively studied including Conway Granite by Billings and co-workers over the years, and a general summary is 44'30'- given by Billings.9 GOrnonI Typical Conway granite is coarse- , grained, pink, and massive and con- ) sists of perthitic microcline, quartz, 4400'- A and minor plagioclase and biotite. Variants, particularly with a green color or finer grain size, occur lo- cally. The Mount Osceola granite 43°30'- is similar in composition and texture to the Conway; the major distin- Concord guishing features of the Mount Os- ceola are the presence of iron-rich 43°00'- olivine and pyroxene and the scar- city of biotite. Contacts between the Mount Osceola and Conway 72°3O* 72W0 713O 7i1OO are apparently gradational, and some confusion between the two FIG. 1.-Index map of New Hampshire showing outcrops of the White Mountain magma series units has undoubtedly occurred, (modified from Billings9). particularly in the field. Method of Quantitative Field Determination of Thorium.-For the purpose of making direct thorium measurements in the field, a portable, single-channel, gam- ma-ray spectrometer (pulse height analyzer) was designed and constructed. The 2.62-Mev gamma ray from the thallium-208 daughter of thorium was detected by means of a 3-inch diameter by 3-inch high Nal (Tl) crystal. This energy level is favorable for measurement because (1) only the thorium series contributes to this high-energy part of the natural rock gamma-ray spectrum, and (2) this high- energy radiation is detected from a large volume of rock, each station being repre- sentative of more than 50 pounds of solid rock. A portable shield (80 pounds) provided approximately 90 per cent protection to the sides and top of the detector from geometric effects and from a minor cosmic-ray component in the narrow win- dow centered on the 2.62-M\ev level. Background was from 3 to 5 cpm (equivalent to 1 to 2 ppm of thorium). This low background and the high thorium content of the rocks studied provided excellent counting statistics for counting periods of 10 minutes or less. A monazite-sand standard was used to calibrate the energy-level discrimination before and after each station. Absolute calibration of the field spectrometer was done by two methods. The first method involved collecting a representative hand specimen at most field sta- tions. The obtaining of a completely representative specimen was difficult be- cause (1) the Conway granite is very coarse-grained, (2) flat horizontal surfaces that Downloaded by guest on October 2, 2021 1900 GEOLOGY: ADAMS ET AL. PROC. N. A. S.

minimized geometric effects for the field instrument maximized the difficulty of sledging out a specimen, (3) the field instrument detects 2.62-Mev gamma rays coming from depths of nearly one foot, and (4) there is some indication that the thorium which is so readily leached from the Conway in the laboratory is also leached from weathered surfaces, thus tending to cause lower thorium concentra- tions in the hand specimens than in the total volume of rock investigated by the field instrument. A calibration curve using the hand specimens of Conway granite for laboratory measurements yielded a constant of 3.19 cpm on the field instrument for each part per million of thorium. A second method of calibration involved (1) the use of a short core taken directly below one of the counting stations and (2) an artificial -sand standard, which formed a large homogeneous system having the density of solid granite. A 10-foot core of 1 '/8 inch diameter was obtained at one station; the average thorium con- tent of this core was determined in the laboratory to be constant with depth ex- cept in the upper two inches, where there had apparently been a slight leaching. In the laboratory, several hundred pounds of zircon sand were arranged to form an essentially infinite system below the field instrument, and the instrument was calibrated against the known thorium content. Use of the average thorium con- tents of the core and the zircon sand gave a constant of 3.04 cpm on the field in- strument per part per million of thorium. This figure is believed to be more accu- rate than the 3.19 figure obtained from hand specimens and is used in the present work. The difference between the two constants is less than the maximum error of 10 per cent estimated on the basis of experiments on geometric effects, cosmic- ray background, counting statistics, and instrumental drift. Complete details on the construction and calibration of the field instrument are given in a forthcoming paper by Adams (see ref. 10). Surficial Survey of the White Mountain Batholith.-In the field, the major effort was in the area of the central White Mountain batholith (Fig. 1). Some reconnais- sance thorium determinations were made on the outlying portions of the White Mountain series, and the concentrations were found to be roughly correlative with those of the units in the main mass. Figure 2 is an outline map of some of the major units of the White Mountain batholith and shows the distribution of counting stations and the location of deep-coring sites. Figures 3 and 4 are histograms of the 2.62-Mev gamma-ray cpm on surface out- crops of the Conway and Mount Osceola granites. As discussed above, slightly more than 3 cpm are equivalent to 1 ppm of thorium. The histograms are quite smooth with the exception of a slight excess of counts in the Conway granite at approximately 130 cpm and a slight excess in the Mount Osceola at approximately 170 cpm. Owing to the fact that the Conway and Mount Osceola granites have modes at 170 and 130 cpm, respectively, it seems likely that in both cases the excess is caused by the indistinct boundaries between the two units and the great difficulty in distinguishing them in the field. A plot on logarithmic probability paper of the cumulative frequency curve of tho- rium concentrations in the main mass of the Conway granite yields an essentially straight line. A small deviation from linearity in the neighborhood of 130 cpm probably represents the inclusion of some stations in the Mount Osceola granite as a result of the difficulty of distinguishing Mount Osceola from Conway granite. Downloaded by guest on October 2, 2021 VOL. 48, 1962 GEOLOGY: ADAMS ET AL. 1901

44015' ,, , + 4415 cg + 4415'

71+454°+44@001

Cl)~~~~~~~~~~~~~~~~C ~~~~~z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~18~~ ~ ~ ~ ~ ~ ~ k~ shown as small dots. Drill sites are labeled A, B, and C. Some of the more important rock types are delineated as follows: cg, Conway granite; mo, Mount Osceola granite; pqs, porphyritic quartz syenite; sy, syenite. Geology com- piled in~~~part10~~~~~~from Billings9 and modified locally by the present writers on the basis840U.of sample'..Pidentifications and field observations. i 0~~~~~~~~~~~~~~~~~~~~~~~~700

ppm Thorium 01450 50 25 10 1050 20 75 20 0100 5 125 0 5150 00' 24 ~~~~~~2~~ ~ ~ ~ ~ ~ ~

c~~~~~~6~ ~ ~ ~ p

CONWAY GRANITE FIG. 3.-Frequency distribution of counts per mma in the thorium channel (2.62 Mev) at 214 surface stations in the Conway granite. All bodies peripheral to the central White Mountain batholith and out- lying bodies from elsewhere in New Hampshire are eliminated from this plot

A deviation from linearity above 220 cpm results from a slight excess of stations with high counting rates; this deviation is believed to result from the field practice of taking more closely spaced stations in the high-counting areas in order to deter- mine the area over which the high-counting rock extends. A lognormal curve drawn as the best visual fit to all of the frequency-distribution data for the main mass of the Conway granite has a chi square for deviation of only 9.27 for 20 de- grees of freedom. The writers conclude that the concentrations of thorium in the main mass of the Conway granite are lognormally distributed. Downloaded by guest on October 2, 2021 1902 GEOLOGY: ADAMS ET AL. PROC. N. A. S.

ppm Thorium The significance of a lognormal dis- 20 0 25 50 75 tribution is as follows: (1) the average oak18- r thorium concentration is best repre- 6 sented by the geometric mean, approx- 4 w~~~~~~~~~~~ imately 170 cpm (56 ppm), rather than =12- by the arithmetic mean of 178 cpm, 0 though the difference between the two 06 rr n means is insignificant in this case; (2) W 4- l a smooth lognormal frequency distri- X2- 1 L m bution is expected for trace ele- Z ments,"1-'3 and a set of measurements 0 50 100 IS0 200 250 which yields such a distribution prob- MT. OSCEOLA GRANITE ably represents adequate random FIG. 4.-Frequency distribution of counts sampling of the rock. Thus, any prac- per min in the thorium channel (2.62 Mev) at 98 surface stations in the Mount Osceola ticable increase in the number of field granite. Only bodies within the central measurements on the surface of the White Mountain batholith are included. Conway granite has little chance of significantly improving the confidence in the average thorium content. It is also possible to argue that a simple lognormal distribution as found in the Conway gran- ite indicates that the thorium is a primary constituent of the rock; a distribution of this type is not found where the thorium is clearly secondary, as in the Enchanted Rock batholith of Texas.i4 The basic conclusions concerning the present accessible surface of the Conway granite are: (1) the upper foot of exposed rock contains an average thorium concen- tration of 56 ppm, and all evidence indicates that the true average lies in the range of 56 i 6 ppm; and (2) additional surface stations are unlikely to change materially the previous conclusion. Thorium Concentrations at Depth in the Conway Granite. Thorium concentrations at depth in the Conway granite have been measured in three AX (1 i/8 inch) cores at the locations shown in Figure 2. Holes were drilled to a depth of 600 feet below the surface at locations A and B and to a depth of 500 feet at location C. By means of a modified portable gamma-ray spectrometer, thorium contents in the cores were measured at approximately 5-foot intervals. A total of 366 measurements was made in the 1,700 feet of available core. No regular changes in thorium concentration with depth were found in any of the three cores. Core C, which started in granite measuring somewhat higher than normal in thorium content at the surface, main- tained that high concentration to its total depth of 500 feet. On the basis of core data, the granite is uniform in thorium content with depth. Compositional uniformity and extent of the Conway granite at depth can be in- ferred from several geologic observations. In the several thousand feet of relief developed on the present Conway outcrop, no relationship has been found between elevation and either texture, bulk composition, or thorium content. Furthermore, streams draining the Conway batholith contain debris almost exclusively derived from granite, indicating a scarcity of other rock types within the plutonic mass. The limited geophysical evidence from gravity studies indicates that the Conway granite contacts are vertical.15 This evidence and studies summarized by Billings9 all suggest that the Conway granite extends as a homogeneous body to a depth meas- Downloaded by guest on October 2, 2021 VOL. 48, 1962 GEOLOGY: ADAMS ET AL. 1903

ured in thousands of feet. Assuming that the thorium con- centration of 56 4- 6 ppm obtains at depth throughout the batholith, the thorium, metal content in the main mass alone is greater than 3 X 106 metric tons per 100 feet of depth. This calculation is based on a total outcrop area of Conway gran- ite of 307 square miles. Sites of Thorium and Uranium in the Conway Granite.-The Conway granite is a low-grade thorium re- source only to the extent that the 0 0.25 mm thorium is readily recoverable. The ease of recovery, in turn, depends upon the sites of thorium. Extrac- tion by leaching with dilute acid, followed by solvent extraction, is be- ing studied at Oak Ridge National Laboratory (personal communica- tions and reports of Chemical De- velopment Section C, Chemical Technology Division). Their results on the Conway granite show that generally 2/3 or more of the thorium FIG. 5.-Photomicrograph (above) and auto- and a slightly smaller fraction of the radiograph (below) of alpha tracks in a thin section of Conway granite. The two photographs are of uranium is leachable, indicating that the same area and show the concentration of radio- these elements are in available sites grains.activity inThesmalldifferent"hot" grainsappearanceenclosedof inthetwotwobiotitebio- to a substantial degree. tite grains in the photomicrograph is caused by dif- The sites of the thorium and ura- ferences in optic orientation. The small radiation flux from the "hot" grains has, in geologic time, nium are readily investigated by the resulted in a sufficient dose to cause a zone of radia- use of alpha-sensitive nuclear emul- tion damage (pleochroic halo) in the biotite around sions. These emulsions may be applied as a liquid directly to uncovered thin sections,'6 or emulsion-covered track plates may be placed in contact with thin sections. The track plates have been particularly successful in studies of the Conway granite, and the resulting autora- diographs demonstrate that most of the thorium and uranium is concentrated in small (50 microns or less) "hot" grains. These "hot" grains are commonly included in biotite, where they form pleochroic haloes and tend to occur in clusters (see Fig. 5). A few "hot" grains have been observed in feldspar. A count was made of 14,400 alpha tracks in an autoradiograph of one thin sec- tion from the 10-foot core. This count showed that 84 per cent of the alpha par- ticles came from areas in or near biotite, 9 per cent from quartz, and 7 per cent from feldspar. Slightly less than 50 per cent of the tracks came from "hot" grains enclosed in biotite. The sources of alpha tracks in quartz were all dis- persed (submicroscopic), but 40 per cent of the tracks in feldspar came from "hot" Downloaded by guest on October 2, 2021 1904 GEOLOGY: ADAMS ET AL. PROC. N. A. S.

grains. This one autoradiograph indicates that between 50 and 80 per cent of the thorium and uranium might be in available sites, which is in good agreement with leaching studies. Billings and Keevil,5 reported allanite as a common carrier of radioactivity in the Conway granite, but a metamict , tentatively identified as thorite, appears to be more important in those samples examined by the pres- ent writers. The uniform, lognormal, concentration of the thorium in the Conway granite, plus its major concentration in "hot" grains such as thorite, indicate that the thorium was probably deposited early during the crystallization of the granite. With 2/3 of the thorium in the Conway granite readily leachable, the recoverable thorium in the granite may be calculated as at least 2 X 106 metric tons per 100 feet of depth in the main mass alone. Reconnaissance measurements indicate that the outlying masses of the Conway granite, as well as other New England granites of the Middle and Late Paleozoic age, are also anomalously high in thorium content. Future Utilization of the Conway Granite.-The costs of extracting the uranium and thorium from the Conway granite are estimated by workers at the Oak Ridge National Laboratory to be less than $100/pound, or at most five to ten times the present costs of nuclear raw materials. This source of nuclear fuels, therefore, is currently uneconomic compared to the sources now being utilized. In terms of total energy content, however, the Conway granite represents an energy resource several orders of magnitude larger than the lower cost material. In the long-term future, when supplies of cheap uranium and thorium may start to be exhausted, sources such as the Conway granite may become increasingly important and neces- sary. In principle, it is possible to produce economic electric power from high- cost thorium by thermal breeding, although to achieve this, many technical prob- lems remain to be solved. Thus the importance of the present work on the Conway granite lies in the in- dication that tens of millions of tons of thorium are available when the need for vast amounts of higher-cost nuclear fuel becomes pressing. These amounts may be compared to the few hundreds of thousands of tons of previously estimated thorium reserves. It is reassuring to know that the long-term future of nuclear power is not limited by the supply or by a prohibitively high cost of fuel. Further- more, the Conway granite may become even more important considering the like- lihood that improved extraction techniques may make the thorium available at costs well below the $100/pound estimated in preliminary laboratory experiments. It is also possible that larger amounts of lower-cost thorium might be realized by locating high-grade ore reserves such as the Lemhi Pass, Idaho, area may prove to bel' or by finding a large granitic batholith more economic than the Conway. To the extent that thorium is essentially as available as uranium, the relative economics and use of the two nuclear fuels will be determined mainly by future research in nuclear-energy systems. Finally, it should be noted that the statistical and exploration techniques devel- oped in the present work and described above, particularly the portable gamma-ray spectrometer, may make it possible to explore for thorium and develop reserves far more cheaply and rapidly than was the case for uranium. Downloaded by guest on October 2, 2021 VOL. 48, 1962 GEOLOGY: W. S. McKERROW 1905

All of the writers wish to acknowledge the support of the Oak Ridge National Laboratorv under Subcontract 1491. Two of us (J. A. S. A. and J. J. W. R.) have received substantial funds under Grant C-009 from the Robert A. Welch Foundation for studies on the geochemistry of thorium. The Robert A. Welch Foundation under Grant K-054b also provided one of us (J. A. S. A.) with funds for both the development and construction of the laboratory and field radiometric equip- ment. The writers would like to thank numerous individuals for their many courtesies and stimulating discussions regarding the present work. In particular, the help of the following is ac- knowledged: A. P. Butler, Jr., D. Gottfried, E. S. Larsen III, and George Phair of the U.S. Geological Survey; K. B. Brown, D. Crouse, and F. Hurst of the Oak Ridge National Laboratory; J. B. Lyons of Dartmouth College; and M. P. Billings of Harvard University. 1 U.S. Atomic Energy Commission, Technical Information Service Report TID-8201 (1959). 2 McKelvey, V. E., Am. J. Sci., 258-A, 234-241 (1960). Adams, J. A. S., J. K. Osmond, and J. J. W. Rogers, Physics and Chemistry of the Earth, 3, 298-348 (1959). 4 Kline, M.-C., J. A. S. Adams, and J. J. W. Rogers, Geological Society of America Special Paper 68, 211 (1961). 5 Billings, M. P., and N. B. Keevil, Geol. Soc. Am. Bull., 57, 797-828 (1946). 6 Whitfield, J. M., Ph.D. Thesis, Rice University (1958). 7 Butler, A. P., Jr., U.S. Geological Survey Professional Paper 424-B, 67-69 (1961). 8 Lyons, J. B., U.S. Geological Survey Professional Paper 424-B, 69-71 (1961). 9 Billings, M. P., The Geology of New Hampshire; Part II-Bedrock Geology: New Hampshire Planning and Development Commission (1956). 10 Adams, J. A. S., Geological Society of America Special Paper 68, 125 (1961). Ahrens, L. H., Geochim. et Cosmochim. Acta, 11, 205-212 (1957). 2 Jizba, Z. V., Geochim. et Cosmochim. Acta, 16, 79-82 (1959). 13 Rogers, J. J. W., International Geological Congress, XXI Session, Part XXI, 275-280 (1960). 14 Ragland, P. C., Ph.D. Thesis, Rice University (1962). 15 Bean, R. J., Geol. Soc. Am. Bull., 64, 509-537 (1953). 16 Guilbert, J. M., and J. A. S. Adams, Nucleonics, 13, 43 (1955). 17 Anderson, A. L., Econ. Geol., 56, 177-197 (1961).

THE CHRONOLOGY OF CALEDONIAN FOLDING IN THE BRITISH ISLES BY W. S. MCKERROW

DEPARTMENT OF GEOLOGY AND MINERALOGY, OXFORD UNIVERSITY Communicated by James Gilluly, August 22, 1962 The Dalradian areas of Scotland and Ireland had their large major folding and metamorphism before the Arenig; strong pre-Silurian folding is known in the Welsh Borderland and western Ireland; at the end of the Silurian, we know there is folding in Berwickshire and Pembrokeshire; in Middle Old Red Sandstone time, the main movements are in South Wales, in the Welsh Borderland, and near fault zones in Scotland. The evidence for these events is provided by unconformities, radioactive age dating of granites and metamorphism, and the oncoming of Old Red Sandstone facies. It is suggested that the Caledonian orogeny is the sum total of all these events from the Upper Cambrian to the Middle Devonian which occur in, or adja- cent to, the Caledonian geosyncline. The stratigraphical evidence of the times of folding is reviewed with the following Downloaded by guest on October 2, 2021