Abstract T he M ining and mineral processing industry is important to the Canadian economy and in 2001 contributed $35.1 billion, or 3.7 percent, to the Gross Domestic Product and employed approximately 376,000 Canadians (Minerals and Metals Sector, Natural Resources Canada). However, over the past decade, Canada’s base metal reserves have declined by more than 25 percent, and significant new discoveries will be required if Canada’s role as a major base metal producer is to be maintained into the twenty-first century. The Bathurst Mining Camp is one of Canada’s most important base metal mining districts, accounting in 2001 for 30 percent of Canada’s production of Zn, 53 percent of Pb, and 17 percent of Ag. In 1999, the Bathurst Mining Camp accounted for 32 percent of the Zn, 80 percent of the Pb, and 25 percent of the Ag reserves (Minerals and Metals Sector, Natural Resources Canada). The value of production from the Bathurst Mining Camp in 2001 exceeded $500 million and accounted for 70 percent of total mineral production in New Brunswick. Approximately 2,000 people are directly employed by the mining industry in the Bathurst Mining Camp. Without the discovery of new ore reserves, however, production will decline and will cease within about 10 yr at current production rates, and with it the principal source of economic activity in northeastern New Brunswick will also disappear. To address the major decline of mineral resources in Canada’s economically important mining districts, EXTECH (Exploration and Technology) projects were established by the Geological Survey of Canada. EXTECH-II is a multidisciplinary, integrated and collaborative project that has focused on the Bathurst Mining Camp with four principal objectives: (1) update and expand the geoscience knowledge base, (2) develop and test new and improved methods of exploring for massive sulfide deposits, (3) conduct ground and airborne, geophysical and geochemical surveys to identify new exploration targets, and (4) build a multiparameter, comprehensive, coregistered, and internally consistent digital geoscience database of the entire Camp. Although EXTECH-II was initiated by the Geological Survey of Canada in 1994, it was a collaborative project involving earth scientists from the Geological Survey of Canada, the Department of Natural Resources and Energy of New Brunswick, universities, and mining and exploration companies. A similar multidisciplinary project was established at about the same time by the U.S. Geological Survey to study the well-preserved Bald Mountain Cu-Zn-Ag-Au massive sulfide deposit in northern Maine. This project, which began in 1995 and ended in 1999, also included selected research on the Mount Chase Zn-Pb-Cu-Ag-Au deposit 70 km to the south of Bald Mountain.

Abstract ARCHEAN Cu-Zn deposits are among the most important mineral deposit types in Canada. The Superior province of Canada contains nearly 80 percent of the known Archean Cu-Zn deposits in the world (about 100 of 125 deposits). These deposits are concentrated in 10 separate mining camps, including Sturgeon Lake, Manitouwadge, Mattagami Lake, Chibougamau, Joutel, Val d’Or, Bous-quet, Noranda, Kidd Creek, and Kamiskotia (Fig. 1 and Table 1). A few deposits in rocks of similar age and composition are also known in the Slave province, the Churchill province, and in the Archean of Western Australia, southern Africa, China, and Brazil. Known deposits of this age worldwide account for more than 650 million metric tons (Mt) of massive sulfides, containing 10 Mt of Cu metal, 29 Mt of Zn, 1 Mt of Pb, 33 Mkg Ag, and 750,000 kg Au. The giant Kidd Creek volcanogenic massive sulfide deposit in the western Abitibi subprovince of Canada is the largest known deposit of this age currently in production. The Superior province is the world’s largest exposed Archean craton, occupying an area of more than 1.5 million km 2 , bounded by the Trans-Hudson orogen to the west and the Grenville province to the east. A number of distinct subprovinces are recognized, assembled into east-west-trending granite-greenstone terranes and metasedi-mentary belts (Fig. 1). The granite-greenstone terranes are composed of gneissic rocks of plutonic origin, supracrustal rocks of dominantly volcanic origin, and a variety of syn- to late kinematic granitoids. Volcanic rocks comprise about 12 percent of the total area. The greenstone belts have been described variously as successive lateral accretions of volcano-plutonic arcs, oceanic islands, oceanic plateaus, and rift-related assemblages (e.g., Langford and Morin, 1976; Percival and Card, 1985; Ludden and Hubert, 1986; Ludden et al., 1986; Card, 1990; Jackson and Sutcliffe, 1990; Williams, 1990; Corfu, 1993; Heather et al., 1995; Jackson and Cruden, 1995). The metallogenic history of the Superior province has been described in detail by Franklin and Thorpe (1982) and Poulsen et al. (1992). The Abitibi subprovince (94,000 km2) is the largest of the greenstone belts. It contains the major gold and base metal mining camps in Canada (Fig. 2), with production and reserves totaling more than 480 Mt of massive sulfide and 4,700 t of Au. Metal production in the western portion of the Abitibi greenstone belt is dominated by the Timmins region, which historically has accounted for 37 percent of the total gold production

Abstract Mineral resources have been important to the Alaskan economy for hundreds of years. The Indians, Eskimos, and Aleuts used gold, copper, and other metals for jewelry, utensils, and weapons and as items of trade. The Russians, who arrived in 1728, showed only minor interest in minerals, focusing instead on furs. Nonetheless, records show that they mined iron ore on the Kenai Peninsula in 1793 and located gold there in 1834. In addition, the Russians were aware of copper-rich occurrences in the Copper River basin. The mining industry grew rapidly in Alaska after the United States purchased the region from Russia in 1867. By 1870, gold was being mined near Windham Bay and near Sitka, in southeastern Alaska. The first major hard-rock gold mining began at the Alaska-Juneau, Perseverance, and Treadwell mines near present-day Juneau following the discovery of placer gold near tidewater in 1880. Significant placer mining operations for gold soon spread northward into districts such as Fairban ks, Nome, Iditarod, and Circle. The exploitation of mineral resources, particularly gold, influenced the settlement and development of much of the state. Major cities such as Nome, Fairbanks, and Juneau, the capital, were originally built around early mining camps. By1996, more than 33 million oz of gold, as well as significant amounts of copper, lead, zinc, silver, and platinum-group elements, had been produced. In addition, there has been minor production of nickel. tin, mercury, and uranium Alaska is presently experiencing a renaissance in mining on federal, state, private, and native lands. From 1990 to 1996, the minerals industly had an annual value of approximately $600 million, with a record 1 billion in 1996. Nearly 3,500 year-round jobs were directly attributed to the minerals industq. Looking forward into the twenty-fi rst centu1y, the outlook is even better. Major mines such as Heel Dog are in production. Recently completed exploration at the Red Dog sedimenta1y exhalative deposit has significantly expanded the reserve base and major mine expansion is contemplated. The Nixon Fork gold skarn deposit was put into production in 1995, with the first ore poured in November. The Greens Creek mine resumed production in late 1996, following renewed exploration and development of the volcanogenic massive sulfide deposit. The Fort Knox gold mine, near Fairbanks, is under construction and is scheduled to yield ore by late 1996. In southwest Alaska exploration success has yielded a significant gold resource at Donlin Creek. In addition, the Kensington-Jualin project is

Abstract Ideas about the genesis of sulfide nickel-copper deposits largely rely on the geology and type of major deposits or new discoveries of the times in which they were proposed. Nickel-copper sulfides in the Norwegian gabbronorite intrusions and in the Sudbury Complex were the early examples supporting the conclusion that mafic intrusions were the main host rocks for sulfide nickel-copper deposits. Experimental work by the Norwegian geochemist J. H. L. Vogt at the beginning of the twentieth century created a firm foundation for the hypothesis of magmatic segregation. However, some of the geologic observations of the Sudbury deposits were in contradiction to the magmatic segregation theory, and many authors considered hy-drothermal processes as the main accumulation mechanism of Sudbury-type Ni-Cu sulfides. These two main theories, magmatic and hydrothermal, still prevail in genetic considerations today. The discovery of shock metamorphic structures around Sudbury led to the idea that the Sudbury Complex was an old meteorite impact site and that the mafic intrusion with Ni-Cu sulfides followed the structure of a meteorite crater. After lively discussion, this idea has been widely accepted, but the type of magmatism, composition of primary melts, assimilation of silicate magma, and emplacement as well as localization of sulfides are still under study. Discoveries of ultramafic-hosted nickel sulfides in Manitoba, Canada, and, later on, the komatiite-associated massive nickel sulfides in Western Australia awakened the discussion of genetic models of nickel sulfides in ultramafic host rocks, which has continued until today. Some studies published in the 1980s indicated that the komatiitic ultramafic flows of the Kambalda area, Western Australia, formed deep thermal erosion channels in the underlying supracrustal sequence, and the lavas assimilated volcanic and sedimentary material. The massive Ni-Cu sulfides formed from the assimilated barren sedimentary sulfides and accumulated at the basal depressions of the komatiitic flows. Experimental studies widened knowledge of the origin of the Ni-Cu sulfides. In the 1960s the sulfide mineral stabilities and composition of sulfide phases were the main targets of study. In the 1970s the sulfide-silicate melt system was examined through the calculation of distribution coefficients between sulfides and silicates. Also, the importance of the magma/sulfide mass ratio (R factor) was discovered. Sulfur isotope studies of the Norilsk deposit, Siberia, proved that sedimentary sulfur was extracted from underlying sediments and reacted with metals in mafic magma to form Ni-Cu sulfide deposits. Since then, the origin of sulfur in Ni-Cu sulfides has been a topic

Abstract The carbonate-hosted ore deposits at Leadville, Gil-man, Red Cliff, Aspen, Alma, Tincup, Kokomo, and Mount Sherman have enjoyed a long and storied production history. These orebodies, as well as dozens of smaller deposits, are all located in the central Colorado mineral belt and together constitute an important metallogenic province (Figs. 1 and 2). Production Recorded metal production of the major districts in this province to date has consisted of 1,630,000 metric tons of zinc, 1,500,000 metric tons of lead, 145,000 metric tons of copper, 15,600,000 kg of silver, and 110,000 kg of gold (Table 1). For several reasons these figures represent only a portion of the metal concentrated by nature in these deposits: 1. Early production records are probably incomplete. 2. Inefficient methods were used to process much of the ore mined during the 1800s, particnlarly for zinc and copper. 3. The ores in the principal mining districts were partially removed by erosion prior to mining. 4. Significant reserves remain in the Leadville district. In comparison to other mining districts around the world, the carbonate-hosted sulfide deposits of the central Colorado mineral belt have produced relatively low tonnages of high-grade ore (Table 2). The largest of the districts is Leadville, which to date has produced aboul 24,000,000 metric tons of polymetallic ore. By contrast, the Aspen district has produced only an estimated 4,000,000 metric tons of ore (Table 2), but that ore averaged about 1,000 g/metric ton silver. Although all of the deposits in this metallogenic province are polymetallic, the economic significance of the various metals is not equal. The ores at Gilman, Aspen, and Leadville were valuable primarily for their contained Zn-Cu-Ag, Ag-Pb, and Ag-Au-Pb-Zn, respectively (Table 2). Discovery The first discovery of gold in Colorado was made in July 1858, in a stream draining the eastern Rocky Mountains. This led to the “Pike's Peak” gold rush of 1859, during which an estimated 50,000 people moved into the area (Blair, 1980). These so-called “Fifty-Niners” established most of the mining districts in the northeast portion of the Colorado mineral belt during the summer of 1859. By late 1859 the prospectors had penetrated the Continental Divide, and in April 1860, the placer gold deposits at Leadville were discovered. A rush to Leadville ensued, and as a result of heavy mining pressure, the Leadville placers were essentially depleted by 1868. The much larger and more valuable carbonate replacement ores at Leadville,

Abstract When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques. There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits. Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in

Abstract This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis. The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.

Abstract This monograph on Magmatic Ore Deposits has resulted from a Symposium held at Stanford University on November 12 and 13, 1966. All except three of the papers that were presented are published in this volume as well as some of the discussion and the summation of the symposium. Unfortunately much of the discussion cannot be included because the volume is already so large. The best introduction to this volume is, perhaps, the introduction as it was presented at the symposium: This symposium was conceived in 1962 when the Program Policy Committee recommended that the . Society of Economic Geologists should sponsor a symposium on magmatic ore deposits. The Committee under the chairmanship of John K. Gustafson believed this to be an effective method of advancing geologic thought. It is fitting that the symposium should finally be held during Gustafson’s presidential year. The proposal of the Program Policy Committee was approved by Council at its meeting in November, 1962. A special committee consisting of G. Kullerud, J. A. Noble, C. H. Smith, T. P. Thayer, with H. D. B. Wilson as chairman, was appointed by the President, Olaf N. Rove, in February 1963 to make arrangements for the symposium. E. N. Cameron, Secretary of the Society, was ex officio member of the special committee and remained as an active member when he resigned the secretaryship. C. H. Park, Jr. joined the committee shortly after its formation. The Program Policy Committee was prompted to recommend the symposium by the realization that the underlying theory of the formation of magmatic ore deposits was formulated many decades ago., In the intervening years, much new data have been acquired from systematic research. It seemed to the Program Policy Committee that it was time for those with an abiding interest in the magmatic deposits to meet to assess this new data and to point out the unresolved problems. The symposium was entitled “Symposium on Magmatic Ore Deposits.” The special committee accepted the terminology in the “Glossary of Geology and Related Sciences,” Edition 2, page 175. Magmatic Deposits Certain kinds of mineral deposits form integral parts of igneous rock masses and permit the inference that they have originated, in their present form, by processes of differentiation and cooling in molten magmas. (Lindgren p. 863, 1929). The symposium committee has added the term “ore” to attempt to keep the discussions centered on ore, or near ore material, or with

Abstract Proponents of syngenetic theory base their interpretation largely on widespread uniform mineralization within a restricted stratigraphic interval and a consistent relationship of mineralization to sedimentary features. Proponents of epigenetic theory base their interpretation on mineralization of post-depositional structures, changes in extent and grade of ore, open space filling, district-wide lack of close control by sedimentary features, and relation of ore to tectonic structures. These and other criteria are evaluated in an attempt to define diagnostic criteria. On the basis of the criteria defined the major lead-zinc deposits of Mid-continent United States must be considered as epigenetic. Features of the Southeast Missouri lead district are listed. The deposits are epigenetic. The metals are believed to have been derived from nearby sedimentary basins and carried out of basins onto shelf areas in a concentrated brine. Movement of solutions was controlled by basement topography and deposition of metals occurred when solutions entered the Bonneterre formation on the flanks of and over buried knobs. Objective .—The problem of origin of stratiform ore bodies cannot be resolved until we define, and agree upon, what constitutes diagnostic evidence for each type of deposit. This paper is an attempt to review the nature of geologic evidence; to define those features that must be regarded as unique and necessary criteria in classifying any deposit or district; and to apply the criteria to a major district, the Southeast Missouri lead deposits. Theories of Origin .—The major elements of theories on origin of stratiform ore bodies are summarized in Table 1. A deposit is Syngcnetic if formed by processes similar to and simultaneously with the enclosing rock; epigenetic if introduced into a pre-existing rock (3). A diagenetic origin implies deposition of metals with the host sediments but with recrystallization, rearrangement, and limited migration. The search for an acceptable theory of origin must be separated into its two component parts: (1) definition of whether the deposit has syngenetic, diagenetic, or epigenetic features and. (2) history of mineralization to explain source, transport, and deposition of metals. A statement of preferred hypothesis is meaningless until the first is answered and accounts for all geologic facts. The answer must be based solely on observed megascopic and microscopic features and on geochemical and isotopic data; it should not be biased by lack of knowledge to answer all phases of the second. In evaluating the evidence to determine type of deposit one cannot be concerned

Abstract In the East Tintic district lenticular beds of tuff and agglomerate are intercalated in the lower part of an Oligocene volcanic series, made up chiefly of porphyritic quartz latite lavas, which was deposited on strongly folded, faulted, and deeply dissected Paleozoic rocks, comprising a thick basement of quartzite and 5,500 feet of dominantly carbonate sediments above. The volcanic series and the Paleozoic formations are cut by intrusive quartz monzonite and porphyritic latites and by pebble dikes —dike-like bodies comprised chiefly of subangular or rounded fragments of quartzite. Hydrothermal alteration has affected all these rocks locally, but ore deposits, though later than the intrusives, have been found only in the rocks below the volcanic series. Most of the ore found in the calcareous sediments is in lead-zinc-silver replacement bodies and most of that found in the quartzite is in pyritic copper-gold veins. The replacement ores commonly have a casing of pyritic jasperoid which in turn is surrounded by an envelope of hydrothermal dolomite. Sericite is abundant close. to the ore and clay-mineral alteration is common locally at the outer edge of the jasperoid. This zonal arrangement is probably the result of different solutions which followed the same path at different times. The stages of alteration include an early, questionably hydrothermal intra-volcanic near-surface alteration, and five subsequent stages of undoubted hydrothermal alteration: (1) the early barren stage; (2) the mid-barren stage; (3) the late barren stage; (4) the early productive stage; and (5) the productive (ore) stage. In the early barren stage, limestone was altered to dolomite and the basal part of the volcanic series was chloritized. The solutions of the early barren stage followed more routes and penetrated farther than those of the later alteration stages, but probably nine times out of ten the ore solutions followed a conduit previously used by the early dolomitizing solutions. Valuable information on the hydrothermal plumbing system was gained through mapping the early barren stage alteration. The mid-barren stage is characterized by the development of clay minerals (argillic alteration) which is most extensive in the volcanic rocks but is also common in the calcareous rocks. Argillization is concentrated around intrusive centers, but the alteration of the intrusives themselves ranges from intense to negligible. A series of alteration zones in the lava may surround partly argillized monzonite bodies; the zones are believed to reflect leaching of the warm intrusive and transfer of material to the cooler country rock during the latter part of the mid-barren stage. Argillic alteration was accompanied by substantial leaching of nearly all the constituents of the rocks and by a corresponding increase in porosity. The solution of the rocks and enlargement of openings constituted an important factor in preparing ground for ore. Later solutions commonly changed the clay minerals into other alteration products along ore channels. The late barren stage alteration is characterized chiefly by jasperoid, barite, pyrite, and sparse chlorite in the sediments and by allophane-quartz, barite, cubic pyrite, calcite, and minor delessite in the overlying volcanic rocks. This alteration is localized near major channels of mineralization and is more closely related to ore than to visible intrusives. Late barren stage silicification seldom reached far beyond the levels of ore deposition; it is meager in the lavas and most extensive in the hydro-thermal dolomite, but pyrite and calcite of this stage traveled much farther. Pyrite tends to concentrate in the volcanics directly above mineralized fissures, but the calcitic replacement is more pervasive and tends to spread out hundreds of feet beyond the pyritic zones. The close association of most of the known ore shoots with late barren stage alteration makes it a favorable indication of proximity to ore. The early productive stage alteration is inconspicuous but almost everywhere present near ore bodies; it is marked by the introduction of potash and minor clear quartz and pyritohedral pyrite. The sericite-hydromica alteration characteristic of this stage seldom extended more than a few feet into the wallrocks of the ore shoots but followed suitable conduits hundreds of feet beyond the upper limits of ore bodies. The productive stage was distinguished by the abundant precipitation of sulfides, sulfantimonides and sulfarsenides, some tellurides and gold, and very little gangue. Weathering of the sulfide ores has given rise to the usual suite of metallic and nonmetallic supergene minerals. Kaolinite, halloysite, jarosite, alunite, and allophane are-prominent where acid sulfate waters have been active, but elsewhere the kaolin minerals and the sulfates are rare. Mont-morillonite, beidellite, endellite, allophane, gibbsite, and calcite are the common products of weathering under the slightly alkaline conditions that exist away from the sulfides. The limonites derived from pyrite, biotite, chlorite, epidote, and magnetite differ in appearance, and this difference greatly aids in the recognition of different types of altered rock. Oxidation of pyrite to limonite is attended by the generation of substantial amounts of heat and an increase in temperature gradient. Both megascopic and microscopic studies of age relations lead to the same conclusions regarding the alteration stages. The mineralogy and paragenesis of the alteration zones change vertically and horizontally away from centers of hydrothermal activity; if arranged according to increasing distance. from these centers, the minerals are also arranged according to increasing age. The genesis and chemistry of an alteration stage is inferred from the paragenesis and the chemical changes that took place. The losses and gains during alteration are summarized. in a series of diagrams, Figures 7,9,10, and 11. During the dolomitic and chloritic alteration of the early barren stage, MgO and FeO increased and CaO, SiO 2 , and A1 2 O 3 diminished. The alteration is assigned to hot chloride solutions containing minor bicarbonates. All constituents of the rocks except water were leached to some extent during the early argillic alteration. These changes are ascribed to halogen acid emanations. In the zones bordering argillized monzonites— which themselves show the normal comprehensive leaching—potash and silica increased. The mineralogy of the surrounding argillic alteration zones is believed to reflect the transfer and redeposition of constituents gained from the hotter monzonite by solutions soaking out into coolor zones. Alunite was probably formed from essentially neutral solutions and the silica and koalin minerals from slightly acid ones. The late barren stage jasperoid replacement bodies represent chiefly deposition of silica and solution of CaO, MgO, and CO 2 from the carbonate rocks and substantial amounts of A1 2 O 3 and K 2 O from the shale. The silica, and accompanying minor barite, and pyrite are ascribed to neutral liquid magmatic bicarbonate solutions carrying an appreciable amount of sulfate and hydrogen sulfide. The presence of manganese on the outer fringe of jasperoidization is probably due to solution and reprecipitation of manganese first introduced during dolomitization. The calcium taken into solution from the dolomites was largely precipitated in the overlying volcanics but magnesium was carried away. The pyritic halos in otherwise unaltered quartz latite, lying over jasperoid, represent no appreciable change in the iron content of the lava and the addition of the sulfur may be due to gas phase transfer of H 2 S. The solutions of the early productive stage added potash to the rocks but did very little else. Minor quartz and pyrite were deposited in open spaces. There is no indication of acid alteration and the abundant sericite formed indicates that the solutions were not strongly alkaline. The ore stage was probably not accompanied by other than sulfide alteration. No alteration thus far recognized is a sure indication of ore. Detailed study of alteration and pebble dikes in the blanket of barren lava has yielded much new information on the structure, alteration, and character of the rocks below. Knowledge of the space relations of altered zones and the stages represented in an area of favorable structure, and the position of the various stages in the sequence of hydrothermal events that culminated in ore deposition, gives a reasonably secure basis for estimating the chance of finding an ore body.

Abstract Studies of strategic mineral deposits in the major domestic pegmatite districts by the U. S. Geological Survey from 1939 to 1945, primarily for the purpose of increasing production of mica, beryllium, lithium, and tantalum minerals, resulted in the clarification of many problems related to the general. features, internal structure, mineralogy, and origin of granitic pegmatites. This report summarizes about 68 man years of work in the pegmatite districts of New England, the Southeastern States, South Dakota, Idaho, Montana, Wyoming, Colorado, and New Mexico. The pegmatites of eastern United States, of Paleozoic age, and those of the Rocky Mountain region, mostly of pre-Cambrian age, are localized in metamorphic terranes, and are closely associated with either granite, quartz monzonite, or granodiorite masses. In some districts where pegmatites are associated with all three types of igneous rocks the composition of the plagioclase feldspar in three separate groups of pegmatites suggests a genetic relationship to the three types of igneous rocks. In other districts the distribution of pegmatites suggests a genetic relationship to the adjacent igneous mass. Consequently the mineralogy, texture, and structure of the pegmatites varies in detail from district to district, though within any one district variations as great or greater may be found as the parent igneous rock is approached. Pegmatites range from a few inches to more than a mile in length, and from a fraction of an inch to more than 500 feet in width. Most are tabular or thinly lenticular; others are branching, irregular, or teardrop-shaped. Pipelike, arcuate, and troughlike forms are less common. The shapes of pegmatites commonly are closely controlled by the type and structure of the wall rock. Many pegmatites in schistose rocks are concordant with regional foliation, and plunge parallel to the linear structure of the schist. The plunge of discordant bodies is commonly controlled by the intersection of fractures. Secondary foliation and drag folds indicate deformation, of the wall rocks by the pegmatites during emplacement. Alteration of schistose country rocks to granulites is most common along the contacts of discordant bodies. The structural and lithologic units that differ in mineralogy, texture, or both have been designated as: (1) fracture filling– tabular units that fill fractures in previously consolidated pegmatite, (2) replacement bodies— units formed primarily by replacement of pre-existing pegmatite, and (3) zones —successive shells, complete or incomplete, around an innermost unit or core that reflect to varying degrees the shape and structure of the pegmatite body. Zones, quantitatively and economically the most important, have been classified as (1) border zones, (2) wall zones, (3) intermediate zones, and (4) cores.