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This report describes the pre-Cambrian, nontitaniferous magnetite district of Clinton County, N. Y. Descriptions of the petrology and structure of individual magnetite deposits and the rocks of the region as a whole are given. The work involved the geologic mapping of the Dannemora, Lyon Mountain, and Churubusco quadrangles, and that part of the Ausable quadrangle that lies north of the Ausable River. Structural interpretations are based on this mapping, on inspection of all the working and abandoned mines in the area, on detailed logging of many thousands of feet of diamond-drill core, and on a study of many new and old detailed dip-needle surveys. Petrological interpretations are based on many hundreds of thin sections from specimens collected during the geological mapping and the logging of diamond-drill-hole core. Interpretation of the magnetite relationships are based on the study of thin sections and polished sections. Previous work in the area and ideas on the origin of the ore and associated rocks are reviewed. Earlier workers have assigned various origins to the magnetite. Some have been in favor of a sedimentary origin, others an igneous origin. Geologists preferring the igneous origin have explained the magnetite as (1) a direct crystal segregation from a magma, (2) the result of reaction with and replacement of older rocks by solutions of magmatic origin, (3) the result of the enrichment in iron of pegmatitic fluids on contact with gabbroic rocks, or (4) the result of pneumatolytic metasomatic replacement. Pre-Cambrian rocks in the area, in order of decreasing age, are the Grenville series (metamorphosed sedimentary rocks), the gabbro-anorthosite complex, Hawkeye granite gneiss, quartz syenite gneiss and the Lyon Mountain granite gneiss complex which includes four distinct units. These are covered in part by. sediments of the lower Paleozoic, and unconsolidated sediments of Quaternary and Recent age. Rocks younger than the pre-Cambrian are discussed only briefly; they include the Paleozoic Potsdam sandstone and the Beekmantown dolomite, and Pleistocene to Recent unconsolidated glacial debris and fluvial sands and gravels. The metasediments of the Grenville series are considered to have been originally calcareous, argillaceous, and arenaceous sediments altered to their present condition by dynamic and contact metamorphism. The gabbro-anorthosites are considered to have been derived by differentiation from a magma. The quartz syenite and the Hawkeye granite gneisses are believed to be different phases produced by the solidification of a magma younger than the gabbro-anorthosite magma. The Lyon Mountain granite gneiss complex is believed to have originated in part by the emplacement of a granite magma, younger than the quartz syenite-Hawkeye, magma, j.nd in part by granitization of magmatic origin. The structure of the area is thought to be the res.ult of regional plastic deformation. Synclines and anticlines of both major and minor dimensions are encountered throughout the region. The folding is in many cases isoclinul and sometimes is overturned. Faults of different magnitudes also exist. All these structures may be deciphered by observation of the foliation and mineral lineation that is typical of all the pre-Cambrian rocks of Clinton County. The ore bodies are confined to the pre-Cambrian Lyon Mountain granite-gneiss complex, the youngest of the preCambrian granite gneisses. The magnetite replaces a series of rocks within this formation, but the most important replacements occur in microperthite granite gneiss, plagioclase granite gneiss, microantiperthite granite gneiss or in pyroxene-contaminated phases of the foregoing rocks. Some of the ore bodies have a plunging cigarlike shoot structure. These shoot structures are connected parallel to the strike of the enclosing rocks by zones of noneconomic magnetite. The ore occurs on the limbs and in the keels of synclines. Many of the magnetite localities examined show the magnetite either replacing rocks with cataclastic textures or associated with rocks, on the hanging or footwall side, having cataclastic textures. The greater part of the magnetite is thought to be of pneumatolytic origin. It is suggested that iron-halogen emanations, derived from magmatic sources, penetrated the Lyon Mountain granite gneiss along sheer zones, and, on hydrolysis, deposited the iron as magnetite. A brief account of the airborne magnetometer survey of this region is included. Traverses at quarter-mile intervals were flown over the magnetite districts. No new anomalies of importance were discovered. INTRODUCTION PUEPOSE AND SCOPE OF THE EEPOET The study of the Clinton County magnetite district was begun during World War II as one of many such investigations in the United States whose purpose was to aid in the development of deposits of strategic minerals. The report is concerned only with the nontitaniferous magnetite. Titaniferous magnetite, except at the Duncan Mountain prospect, is not found in the area. Work was started late in the field season of 1944 and was resumed during the field seasons of 1945 GEOLOGY OF CLINTON COUNTY MAGNETITE DISTRICT, NEW YORK and 1947. In the course of the investigations, two heretofore geologically unmapped quadrangles, Dannemora and Churubusco, were completed, the Lyon Mountain quadrangle was remapped, and part of the Ausable quadrangle was remapped to provide additional structural and petrological information. The area was mapped on the standard (1/62,500) topographic base; in addition, a few small areas were mapped on enlargements (2 to 3 times) of the 1/62,500 base. The geologic mapping, investigation of diamond-drill cores, and general petrographic study were supplemented by examination of approximately 675 thin sections and 30 polished specimens of ores. Approximately four winters were devoted to laboratory investigation of petrographic material and to compilation of the report and maps. The purpose of this report is to assemble all previously existing information on the region and to add the new information that has resulted from this further field and laboratory investigation. New investigation, both in the field and in the laboratory, was guided by recent trends in geologic philosophy and technique. LOCATION AND CULTURAL FEATURES Clinton County is the extreme northeastern county in New York State. Its neighboring counties to the west and south are Franklin and Essex, respectively. The eastern boundary is Lake Champlain. The international boundary between the United States and Canada forms the northern limit. Plattsburg is the county seat. The area covered in the present investigation is shown in figure 1. The region is dominantly agricultural. Dairy products, fruit, and potatoes represent the bulk of the farm output. Pulpwood is cut and marketed from the wooded sections. Manufacturing industries are confined to Plattsburg and vicinity. Electric power is generated at several points along the Saranac River. An excellent road network permits access to the greater part of the area. The main roads, including secondary roads, are kept open throughout the winter. Timber trails lead to most of the rugged, forested sections though local knowledge is necessary for their use. The main line of the Delaware and Hudson Railroad crosses the eastern part of Clinton County from north to south; a spur line is operated from Plattsburg to Lyon Mountain; another spur connects Plattsburg with Ausable Forks. The Rutland Railroad gives rail service to the northern part of the county. TOPOGRAPHY AND DRAINAGE Maximum relief in Clinton County is approximately 3,710 feet. The highest altitude is on Lyon Mountain, 3,830 feet; the lowest altitude is at the shore line of Lake Champlain, somewhat under 120 feet. The area may be divided into two topographic units: a flat, gently rolling unit of low relief and a rugged mountain unit of high relief. The flat unit extends across the entire northern third of the county and projects southward along the shore line of Lake Champlain and westward from the lake for 10 to 15 miles. This is part of the St. Lawrence Valley physiographic province. The remaining southwestern part of the county is in the northeastern sector of the mountainous Adirondack province. Known zones of magnetite deposits are restricted to the mountain zone. The major mountain blocks trend north, though some blocks have pronounced easterly trends. Local differential relief ranges from about 800 feet to 2,200 feet. The most important valleys are alined east and west. Drainage in Clinton County empties into Lake Champlain and is controlled by four master streams whose dominant line of flow is eastward. These streams are from north to south, the Big Chazy, Saranac, Salmon, and Ausable Rivers. Innumerable small streams feed these main trunks. Small lakes and ponds abound in the region. Two large lakes, Chazy and Upper Chateaugay, should be given specific mention; they lie on the boundary between the Lyon Mountain and Churubusco quadrangles. PREVIOUS WORK IN THE AREA Ebenezer Emmons (1842) has presented one of the earliest accounts of the iron-mining industry in Clinton County. His work is of interest to those concerned with the historical aspects of New York State iron; it is also a valuable guide to those interested in reexamining old workings. Additional historical reviews have been written by Witherbee (1916) and Linney (1943). Geologic studies of the northeast Adirondack 'iron ores include the work of Newland (1908). Nason (1922) described the Adirondack ores and gave an estimation of ore reserves by districts. Two quadrangles adjacent to the Dannemora quadrangle, the Ausable quadrangle (Kemp and Ailing, 1925) to the south and the Lyon Mountain quadrangle (Miller, 1926) to the west, have been mapped. Reports on these two quadrangles include descriptions of the important mines in the region. Ailing (1939) in another paper presented his opinion on the

Geology of Clinton County magnetite district, New York