The Inventario Minero Nacional (IMN), a four-year cooperative geologic mapping and mineral resources appraisal project, was accomplished under an agreement between the Republic of Colombia and the U. S. Agency for International Development from 1964 through 1969. Subzone IIB, consisting essentially of the east half of Zone II, o comprises nearly 20,000 km , principally in the Department of Antioquia, but including also small parts of the Departments of Caldas and Tolima. The rocks in IIB range from Precambrian to Holocene. Precambrian feldspar-quartz gneiss occupies a mosaic of fault-bounded blocks intruded by igneous rocks between the Otu fault and the Rio Magdalena. Paleozoic rocks are extensive, and include lightly metamorphosed graptolite-bearing Ordovician shale at Cristalina, and a major suite of graphitic quartz-mica schist, feldspathic and aluminous gneiss, quartzite, marble, amphibolite, and other rocks. Syntectonic intrusive y.neiss intruded manv of the older rocks during a late Paleozoic(?) oiojM-ny, wliidi war; accompanied l>y Abukiuna-t ype me I atnorph i sni ranj\ii\j\ from lowermost groenschisl to upper amphibolite facies. A Jurassic diorite pluton bounded by faults cuts volcanic rocks of unknown age east of the Otu fault. Cretaceous rocks are major units. Middle Cretaceous carbonaceous shale, sandstone, graywacke, conglomerate, and volcanic rocks are locally prominent. The Antioquian batholith (quartz diorite) of Late Cretaceous age cuts the middle Cretaceous and older rocks. A belt of Tertiary nonmarine clastic sedimentary rocks crops out along the Magdalena Valley. Patches of Tertiary alluvium are locally preserved in the mountains. Quaternary alluvium, much of it auriferous, is widespread in modern stream valleys. Structurally IIB constitutes part of a vast complex synclinorium intruded concordantly by syntectonic catazonal or mesozonal felsic plutons, and by the later epizonal post-tectonic Antioquian batholith. Previously unrecognized major wrench faults are outstanding structural features of IIB. Some are traceable for several hundred kilometers and probably have displacements measurable in kilometers, although only the Palestina fault, with right-lateral displacement of 27.7 km, is accurately documented. Correlations of rocks mapped in IIB with those of outlying areas including neighboring IIA are discussed.

This report summarizes the geology of an area of some 6000 square kilometers in the northern part of the Central Cordillera of the Colombian Andes. The area, in north-central Department of Antioquia, was mapped between 1964 and 1968 as part of the Inventario Minero Nacional (IMN) project. Mineral resources are summarized Mthin a larger area, designated as subzone IIA of IMN Zone II, which comprises almost 22,000 sq. km, including the area mapped geologically by IMN and additional areas mapped by other agencies. The oldest formation is a micaceous paragneiss of early Paleozoic or possibly late Precambrian age. A thick geosynclinal sedimentary series accumulated during the Paleozoic Era and became regionally metamorphosed to greenschist (locally amphibolite) facies during the Permain or early Triassic; these schists and gneisses are designated collectively as the Valdivia Group. The Permian(?) orogenic episode included intrusion of concordant syntectonic plutons, mostly of tonalitic composition. Rocks of unequivocal Triassic or Jurassic age are not recognized. Mineral resources in subzone IIA are varied but not of outstanding importance. Gold and silver mining, significant in past centuries, is minor today. Ferruginous laterite on serpentinite once considered as a potential source of iron ore is not economically exploitable. IMN has explored nickeliferous laterite at the extreme northwest corner of subzone IIA; this is a potential resource, exploitable only after exhaustion of the larger and richer nickel laterite deposit at Cerro Matoso, farther to the north and outside the boundaries of Zone II. Known deposits of mercury, chromium, manganese, and copper are small, with limited economic potential. Nonmetallic resources include raw materials for cement, including portland cement. Saprolite clay is widely used in making common red brick and tile, still a dominant construction material in all but the most modern multistory buildings. Aggregate materials are varied and abundant. Kaolin of good quality near La Unio'n is important as a ceramic raw mineral filler. Tertiary subbituminous coal beds are an important energy resource in western subzone IIA, and have a good potential for greater development. Deposits of sodic feldspar, talc, decorative stone, and silica are exploited on a small scale. Chrysotile asbestos deposits north of Campamento are being developed to supply fiber to Colombia's thriving asbestos-cement industry which previously had been dependent on imported fiber. fire assays for gold and silver were made in the laboratory in Medellin under the successive direction of Drs. Guillermo Serna, Fedro Hernandez, and Leonardo Restrepo. Chemist Jorge Vasquez made many analyses of nickeliferous laterite samples. Taissir Kassem's photointerpretations greatly aided the field mapping. Some of the staff geologists helped in data compilation; Humberto Gonzales and Antonio Erazo helped with thin section studies, including point-count modal analyses; Hernando

of Reserves. 4 Mon-metallic minerals Limestone 5 Marble " ? Dolomite Talc-tremoiite 3 Clay ar.cl feldspar. 8 Metallic Minerals Deposits of iron^ titanium and apatite 9 T^ * O ^N "^-/^ o * ^t") "i /^» T^ /T"" i^ o v»-1 o jL'.'-.v; 1.2-i.ilC ivliuc;! c;ib Iron 39 Copper 43 Zinc 43 Lead 44 References 44

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The mineral resources of an area of 40,000 sq km, principally in the Department of Antioquia, but including small parts of the Departments of Caldas, Cordoba, Risaralda, and Tolima,were investigated during the period 1964-68. The area is designated Zone II by the Colombian Inventario Minero Nacional(IMN). The geology of approximately 45 percent of this area, or 18,000 sq km, has been mapped by IMN. Zone II has been a gold producer for centuries, and still produces 75 percent of Colombia's gold. Silver is recovered as a byproduct. Ferruginous laterites have been investigated as potential sources of iron ore but are not commercially exploitable. Nickeliferous laterite on serpentinite near Ure in the extreme northwest corner of the Zone is potentially exploitable, although less promising than similar laterites at Cerro Matoso, north of the Zone boundary. Known deposits of mercury, chromium, manganese, and copper are small and have limited economic potential. Cement-raw materials are important among nonmetallic resources, and four companies are engaged in the manufacture of portland cement. The eastern half of Zone II contains large carbonate rock reserves, but poor accessibility is a handicap to greater development at present. Dolomite near Arnalfi is quarried for the glass-making and other industries. Clay saprolite is abundant and widely used in making brick and tiles in ***' small backyard kilns. Kaolin of good quality near La Uni6n is used by the ceramic industry. Subbituminous coal beds of Tertiary age are , \ * ' ' ' ' $ k an important resource in the western part of the zone and have good I ' * 'V;< ** potential for greater development. Aggregate materials for cdn*truction ,t-: t ,; are varied and abundant. Deposits of sodic feldspar, talc, decorative stone, and silica are exploited on a small scale. Chrysotile asbestos ' "-" r \\.^ .-^t.;.-d eposits north of Campamento are being developed to supply fiber foe ' vV. __. Colombia's thriving asbestos-cement industry, which is presently dependent upon imported fiber. Wollastonitie and andalusite are-potential