Cerro Mercado

Durango, Mexico

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Cerro Mercado is a Tertiary, sub-aerial volcanic associated, magnetite-apatite iron deposit located on the northern outskirts of Durango City, in Durango State, Mexico.
(#Location: 24° 3' 19"N, 104° 40' 21"W).

The Cerro de Mercado deposits are enclosed within a sequence of silicic felsic volcanic rocks of the Chupaderos caldera, within the 34 to 27 Ma Sierra Madre Occidental volcanic province of central western Mexico. This large silicic volcanic province has NW-SE elongated dimensions of ~2000 x 200 to 500 km, making it one of the world's largest accumulations of felsic volcanic rocks. The Sierra Madre Occidental volcanic sequence is bimodal, but is dominantly felsic (Bryan et al., 2008). The volcanic rocks of the Chupaderos caldera overlie a 31 Ma unit of welded silicic tuffs. These in turn overlie 48 Ma andesites and limited outcrops of Cretaceous shelf limestones, part of the preceding regional Late Cretaceous to Eocene (~100 to ~41 Ma; McDowell and Clabaugh, 1981; Molina et al., 2008) contractional, subduction related calc-alkaline magamatism above the Farallon oceanic plate being subducted toward the NE below the North American continental plate. There is a marked regional break in magmatism between the two volcanic suites over the period from ~41 to 34 Ma (McDowell and Clabaugh, 1981). The youngest rocks overlying all of these are the 12 Ma Metates basalt, extruded during a phase of extensional faulting.

Bryan et al. (2008) suggest that, based zircon U-Pb ages, trace element compositions and mass balance considerations, the extensive younger 34 to 27 Ma explosive rhyolitic volcanic rocks of the Sierra Madre Occidental volcanic province are predominantly the result of melting of existing crustal rocks, largely composed of the underlying subduction-related Late Cretaceous to Eocene andesitic pile, but also Proterozoic crustal basement, with some components of fractional crystallisation and assimilation of primitive, mantle derived magmas. They interpret their data to attribute the crustal melting to long-lived heat and material flux of underplated mantle-derived mafic magmas (reflected by the contemporaneous mafic component of the bimodal Sierra Madre Occidental suite) introduced during continental extension that post-dated the conclusion of subduction-related contractional tectonics (at ~44 to 41 Ma; Molina et al., 2008), and eventually led to the opening of the Gulf of California in the Late Miocene. Ferrari et al., (2005) attribute the mantle underplating and extension to the progressive southward detachment of the Farallon oceanic slab, following the cessation of its subduction, and the possible resultant asthenopheric inflow into the mantle wedge, promoting patial crustal melting that sourced the large scale rhyolitic ignimbrites and lesser mafic volcanism of the Sierra Madre Occidental igneous province.

The mineralisation at Cerro Mercado was emplaced during a sub-aerial volcanic phase in a hiatus between two major eruptive cycles in the Chupaderos Caldera, which is located at the intersection of two fault systems with north-south and NE-SW orientations. The first major eruption from the caldera produced hematitic rhyolite ash-flow tuffs of the Aquilar Formation. Subsequent resurgent doming of the caldera floor filled the moat around the central dome of the caldera with the Cacaria Formation. The latter formation commenced with the Leona Member, a lower unit of rhyolite flow domes, flows and volcaniclatic tuffs. Both the Cacaria and Leona Member are overlain by the facies of the Mercado Iron Member. The succeeding second major stage of volcanic activity comprises the welded tuffs of the Santuario formation which includes fragments of iron oxide in its basal sections.

The Mercado Iron Member is composed of 7 facies, as follows:  i). Martite facies at the base of of the member, with dyke and plug-like extensions into the underlying rhyolite - massive to layered, coarsely crystalline, porous martite (hematite pseudmorphs after magnetite);  ii). Sandy magnetite facies occurring above the martite facies - unconsolidated, laminated fine grained sandy magnetite;  iii). Blocky facies - unlaminated sandy magnetite matrix mixed with blocks of the overlying quartz latite flows;  iv). Mixed iron oxide facies - occuring as tabular and dykelike bodies of fine-grained magnetite-hematite intergrowths that cut and cap the member.

An associated satellite deposit at Peña Morado is composed of a futher 3 facies:  v). Breccia facies of dense, fine grained hematite at the base with included fragments of rhyolite porphyry;  vi). Layered facies of dense fine-grained hematite interlayered with laminated hematite powder; and  vii). Powdery hematite facies of very fine grained, finely laminated crystalline hematite powder at the top.

The main Cerro de Mercado deposit is interpreted (Lyons, 1988) to be the result of the eruption of a iron-oxide magma rich in fluorine, chlorine, carbon dioxide and water, with sheeted flows and flow breccias forming a volcanic dome above an intrusive feeder system. The iron oxides were crystallised as magnetite, concurrently accompanied by abundant, clear, yellow-green apatite crystals in gas cavities and open breccias. The same author suggests large volumes of halogen-rich gases streamed up through the iron oxide flows and oxidised the magnetite to hematite (martite) and redeposited the iron leached from the now-porous martite as laminated sandy magnetite in an extensive fumarolic blanket. A late stage quartz latite dyke intruded and flowed out over the deposit, producing basal blocky flow breccias in which 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. Subsequent late-stage hematite-magnetite dykes cut the entire system and poured out to the surface as flows that capped the mound.

The volatile-rich nature of the mineralising system extensively replaced the underlying pre-mineralisation rhyolites with a mixed assemblage of magnetite and pyroxene, while post-mineralisation tuffs that overlie the iron ore contain a basal layer containing iron oxide fragments, with no associated alteration. The main deposit is surrounded by an aureole of argillic alteration (Megaw & Barton, 1999).

REE at Cerro de Mercado are mainly hosted by fluorapatite, which occurs intergrown with Fe oxide (martite, pseudomorphs after magnetite), lining cavities, fractures, and bedding planes within the main ore body (as described above; Lyons, 1988; Gleason et al., 2000). The REE reach higher concentrations in paragenetically late apatite.

Lyons (1988) suggests an immiscible iron-rich volatile phase evolved from the parent rhyolite magma, formed by the introduction of CO2 into the magma through assimilation of the carbonate wall rocks. This volatile-rich phase then rose to the top of the magma chamber and up through the resurgent floor of the caldera.

The deposits of the Mercado Iron Member outcrop sporadically over an area of 300 km
2, wherever the contact between the Cacaria and Aquila formations is exposed.   The Cerro Mercado deposit lies within a 0.5 km2 right angled triangle, the northern edge of which is exposed by erosion. The western edge is buried by overlying lavas, while the south-eastern margin is defined by a fault. The deposit is from 100 to 200 m thick where best developed, thinning to 10 to 20 m on its margins. Underlying the ore there is a complex net of anastomosing iron oxide veins and dyke-like bodies.

Regionally the stratigraphic position of the ores is represented by peripheral deposits up to 1 m in thickness to zones of hematite staining at the contact.

REE within the ores at Cerro de Mercado are mainly hosted by fluorapatite, which occurs intergrown with iron-oxide (martite, pseudomorphs after magnetite), lining cavities, fractures and bedding planes within the main ore body, and has higher concentrations in paragenetically late apatite (Lyons, 1988).

During the period of peak production in the mid 1970's, up to 1 Mt of ore were produced per annum at grades of 65 to 70% Fe as 90 to 95% iron oxide with accessory apatite, diopside-hedenbergite, quartz and calcite.

The most recent source geological information used to prepare this summary was dated: 2008.     Record last updated: 14/12/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Lyons J I  1988 - Volcanogenic iron oxide deposits, Cerro de Mercado and vicinity, Durango, Mexico: in    Econ. Geol.   v83 pp 1886-1906

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