Abstract Porphyry Cu and porphyry Mo deposits are large to giant deposits ranging up to >20 and 1.6 Gt of ore, respectively, that supply about 60 and 95% of the world’s copper and molybdenum, as well as significant amounts of gold and silver. These deposits form from hydrothermal systems that affect 10s to >100 km 3 of the upper crust and result in enormous mass redistribution and potential concentration of many elements. Several critical elements, including Re, Se, and Te, which lack primary ores, are concentrated locally in some porphyry Cu deposits, and despite their low average concentrations in Cu-Mo-Au ores (100s of ppb to a few ppm), about 80% of the Re and nearly all of the Se and Te produced by mining is from porphyry Cu deposits. Rhenium is concentrated in molybdenite, whose Re content varies from about 100 to 3,000 ppm in porphyry Cu deposits, ≤150 ppm in arc-related porphyry Mo deposits, and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-type) porphyry Mo deposits. Because of the relatively small size of porphyry Mo deposits compared to porphyry Cu deposits and the generally low Re contents of molybdenites in them, rhenium is not recovered from porphyry Mo deposits. The potential causes of the variation in Re content of molybdenites in porphyry deposits are numerous and complex, and this variation is likely the result of a combination of processes that may change between and within deposits. These processes range from variations in source and composition of parental magmas to physiochemical changes in the shallow hydrothermal environment. Because of the immense size of known and potential porphyry Cu resources, especially continental margin arc deposits, these deposits likely will provide most of the global supply of Re, Te, and Se for the foreseeable future. Although Pd and lesser Pt are recovered from some deposits, platinum group metals are not strongly enriched in porphyry Cu deposits and PGM resources contained in known porphyry deposits are small. Because there are much larger known PGM resources in deposits in which PGMs are the primary commodities, it is unlikely that porphyry deposits will become a major source of PGMs. Other critical commodities, such as In and Nb, may eventually be recovered from porphyry Cu and Mo deposits, but available data do not clearly define significant resources of these commodities in porphyry deposits. Although alkali-feldspar rhyolite-granite porphyry Mo deposits and their cogenetic intrusions are locally enriched in many rare metals (such as Li, Nb, Rb, Sn, Ta, and REEs) and minor amounts of REEs and Sn have been recovered from the Climax mine, these elements are generally found in uneconomic concentrations. As global demand increases for critical elements that are essential for the modern world, porphyry deposits will play an increasingly important role as suppliers of some of these metals. The affinity of these metals and the larger size and greater number of porphyry Cu deposits suggest that they will remain more significant than porphyry Mo deposits in supplying many of these critical metals.

A geological map of the Caledonian orogen in East Greenland at the scale of 1:1,000,000 accompanies this volume. The map sheet is a compilation of lithostructural data, and it includes cross sections and inset synoptic tectonic maps with profiles. The ∼1300 km length of the N-S–trending Caledonian orogen in East Greenland is divided into three lithostructural domains—the Caledonian foreland, which is partly exposed in the west, a western marginal thrust belt with foreland windows exposed in anticlinal culminations, and an eastern thick-skinned thrust belt that incorporates major segments of reworked Laurentian gneiss basement in major thrust sheets. Caledonian migmatites and granite intrusions are widespread in the southern part of the orogen. Transport directions of the major thrusts are to the west-northwest, and restoration indicates total displacements on the order of 200–400 km, with estimated shortening of 40%–60%. Archean and Paleoproterozoic gneiss complexes, reworked during Caledonian orogenesis, are widespread. In the south, they are overlain by late Mesoproterozoic metasedimentary rocks. Overlying Neoproterozoic and Lower Paleozoic sediments laid down at the margin of Iapetus reach ∼20 km in thickness. In the north, the Paleoproterozoic basement gneisses are overlain by late Paleoproterozoic to early Mesoproterozoic quartzites interbedded with basaltic rocks and cut by doleritic dikes and sills. Overlying Neoproterozoic and Lower Paleozoic sedimentary rocks relate to developments on the south side of the Franklinian Basin, which extends across North Greenland and into Arctic Canada.

The East Greenland Caledonides, which make up an ∼1300-km-long stretch of North-East Greenland, were formed by the collision of Laurentia and Baltica in mid-Silurian time. Geological mapping and research in this remote and poorly accessible segment of the circum-Atlantic Caledonide orogen began in connection with geographical exploration voyages in the early part of the nineteenth century. The first regional geological mapping took place during the long series of “The Danish Expeditions to North-East Greenland” between 1926 and 1958. Modern geological research and regional mapping by the Geological Survey of Denmark and Greenland between 1968 and 1998 have resulted in the publication of a series of 1:500,000 geological maps of the orogen, and an overview geological map at 1:1,000,000 scale, which accompanies this volume. This article reviews the history of geological research and the evolution of interpretations of the orogen. The recent systematic studies by the Geological Survey of Denmark and Greenland supplement and build on the considerable existing published literature and demonstrate that the North-East Greenland segment of the Caledonide orogen consists of a westward-propagating thrust sheet pile, with displacements estimated at 300–500 km. The thrust sheets incorporate major segments of reworked Laurentian gneiss basement, and a thick succession of Neoproterozoic to Ordovician sediments that accumulated in a major basin originally located outboard of the present coastline.