The iron deposits of the Durango City, Mexico, area were formed by subaerial volcanic processes during a hiatus between two major eruptive cycles emanating from the 30-m.y.-old Chupaderos caldera. The first major eruption of the Chupaderos caldera produced the hematitic rhyolitic ash-flow tuffs of the Aguila Formation. During resurgent doming of the caldera floor, the Cacaria Formation filled the moat around the central dome of the Chupaderos caldera. The lower Cacaria, the Leona Member, consists of extensive rhyolite flow domes, flows, and volcanoclastic tuffs. The various facies of the Mercado Iron Member were deposited on the surface of the Leona Member as well as on the resurgently domed Aquila Formation. A minor amount of quartz latite extrusive activity was concurrent with the eruption of the Mercado Iron Member. Both units preceded the eruption of a second major welded tuff, the Santuario Formation, which incorporated fragments of iron oxides in its base.The Mercado Iron Member of the Cacaria Formation consists of seven facies variations. The Cerro de Mereado iron deposit consists of four facies: (1) a martitc facies--massive to layered coarsely crystalline porous martitc (hematite pseudomorphous after magnetite) at the base, with dike and pluglike extensions downward into the underlying rhyolite, (2) a sandy magnetite facies--unconsolidated, laminated fine-grained sandy magnetite above the martite, (3) a blocky facies--unlaminated sandy magnetite matrix, mixed with blocks of the overlying quartz latite flow, and (4) a mixed iron oxide facies--tabular and dikelike bodies of fine-grained magnetite-hematite intergrowths that cut and cap the system. A satellite deposit at Pena Morada consists of three facies: (5) a breccia facies--dense, fine-grained hematite at the base with included rhyolite porphyry fragments, (6) the layered facies--dense fine-grained hematite layers interlayered with laminated hematite powder, and (7) the powdery hematite facies--a very fine grained cyrstalline hematite powder at the top which is finely laminated. The dense layers of the middle facies (6) exhibit crackled, vesicular, and ropy textures on their upper surfaces. The powdery hematite facies (7) also forms an extensive blanket of laminated, hematite powder referred to as the peripheral deposits. These deposits discontinuously crop out over an area of 300 km 2 around the Cerro de Mercado and Pena Morada deposits, wherever the contact between the Leona Member and the Aguila Formation, and the younger units is exposed. The peripheral deposits vary from 1 m in thickness to little more than staining of the contact. Regionally they thin away from Pena Morada, while locally they are thinnest on palcotopographic slopes.Geologic relationships suggest that the iron deposits formed as a result of a variety of subaerial volcanic processes. The main deposit at Cerro de Mereado apparently resulted from the eruption of an iron magma rich in fluorine, chlorine, carbon dioxide, and water. Sheeted flows and flow breccias formed a volcanic dome above an intrusive feeder system. Iron oxides crystallized as magnetite, with abundant, clear, yellow-green apatite crystals forming concurrently in gas cavities and open breccias. Large volumes of halogen-rich gases streamed up through the iron oxide flows and oxidized the magnetite to hematite (martite) and redeposited the iron leached from the now-porous martite as laminated sandy magnetite in an extensive fumarolic blanket. During the later stages of the cooling process, a quartz latite dike intruded and flowed out over the deposit. Basal blocky flow breccias of the quartz latite mixed with and disrupted the finely laminated texture of much of the sandy magnetite, creating extensive quartz latite breccias with a sandy magnetite matrix. Late-stage hematite-magnetite dikes cut the entire system and fed flows which capped the mound. Lateral to Cerro de Mereado large volumes of iron-rich vapor explosively vented into the atmosphere and crystallized as fine-grained hematite dust which formed an ashlike blanket covering an area greater than 300 km 2 . Flow textures, interlayered with the ashlike hematite, at the base of the Pena Morada deposit, suggest actual flows or welded flows of this material. The occurrence of a maximum thickness of the ashlike hematite at Pena Morada indicates its proximity to a vent. At Cerro de Mercado the volatile-rich nature of the system resulted in extensive replacement of the underlying premineralization rhyolites by a mixture of magnetite and pyroxene. Postminer-alization tuffs overlying the iron ore contain iron oxide fragments at their bases with no alteration.An immiscible iron-rich volatile phase is believed to have evolved from the parent rhyolite magma by the introduction of CO 2 into the magma from carbonate wall rocks. This volatile-rich phase rose to the top of the magma chamber and up through the resurgent floor of the caldera. The fluid is believed to have boiled during its ascent, separating into vapor and liquid phases. At the magmatic temperatures envisioned, water would disassociate and the oxygen would combine with the iron in the liquid phase to form a volatile-rich iron oxide magma which was driven to the surface by a continuing stream of escaping gases. The hydrogen escaped in the vapor phase along with chlorine and fluorine, forming an intensely acid environment capable of carrying significant volumes of iron in the form of iron chloride vapors until reaching the atmosphere where the microcrystalline hematite powder was formed.Comparison of Cerro de Mercado with other apatite-bearing, low titanium iron deposits associated with silicic volcanic systems suggests that this volcanogenic model may be applicable to many of them. The volcanic environment produces a mixture of intrusive, replacement, and sedimentary textures which may explain the heated debates found in the literature over the origin of many of these deposits. These systems include the Kiruna deposits of Sweden, the central Missouri iron deposits, and the Olympic Dam deposit, all of Precambrian age; the Jurassic deposits of northeast Nevada; and the Tertiary deposits of Mexico and Chile.

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