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Ojuela

Durango, Mexico

Main commodities: Ag Zn Au Pb
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The Ojuela carbonate replacement silver-lead-zinc-gold deposit is located at Mapima in the state of Durango in northern Mexico. It lies between the deposits of Chihuahua such as Santa Eulalia and Naica, to the north-west ,and those of Zacatecas, such as Fresnillo and Real de Angeles, to the south-east.

Ojuela is a historic producer, having yielded 6 Mt, of ore, comprising the following ore types (Prod.+Res., 1987, Megaw, et al., 1988):
  Chimney ore - 500 g/t Ag, 10% Pb, 15% Zn, 3 g/t Au (Av. grade, Megaw, et al., 1988);
  Manto ore - 200 g/t Ag, 8.5% Pb, 11% Zn, (Av. grade, Megaw, et al., 1988).

Regional Setting

For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to the deposits of that belt, see the Regional Setting section of the Fresnillo record.

Geology

Like many of the similar deposits of the Mexican silver belt, Ojuela lies within the Mexican Thrust Belt of the Sierra Madre Oriental, at the boundary with the Sierra Madre Occidental volcanic plateau to the west. It is however on the southern edge of the Transverse Range section of the thrust belt (Megaw, et al., 1988). Although Ojuela is similar to the Santa Eulalia orebodies, it is hosted by a well deformed sequence, in contrast to the gently dipping succession at the latter.

Mineralisation is hosted by a Mesozoic sequence that commences with at least 300 m of sandstone, overlain by a transition zone through interbedded shales to dark shaly limestone and finally pure limestone. The limestones may be 2000 m thick. Towards the centre of the section there are two shale bands, each from 30 to 60 m thick, separated by 100 m of limestone. The sediments are intruded by irregular intermediate bodies which are accompanied by calc-silicate alteration. The calc-silicates, chiefly garnet, hedenbergite and vesuvianite (Megaw, et al., 1988), are not physically associated with the orebodies, being centred to the north and east of the mineralisation (Prescott, 1926). In addition, deep drilling has encountered an alaskite body at a depth of 1500 m below the orebody, while deep erosion in the area has exposed a granite mass in the core of the ranges in which Ojuela lies (Prescott, 1926). This granite is approximately 3 km from the mineralisation. It has been suggested that the calc-silicates are associated with these pre-mineralisation intrusives (Megaw, et al., 1988).

Structure

The structure of the district is quite complex, with the host limestones being contorted into a series of folds that strike NW-SE, accompanied by over-thrusting, and separated one from the other by high angle parallel faults with vertical displacements of hundreds of metres.

Mineralisation

The orebodies are found within limestones, below the shales that occur towards the middle of the limestone sequence. In a few instances the mineralisation reaches the shales where it ‘mushrooms out’, while occasionally, sub-economic mineralisation passes through the shale into the overlying limestones. The mineralisation is selective in that it is preferentially associated with dolomitic beds, but is seldom found in shale bands (Prescott, 1926).

Ore occurs as both chimneys and as mantos. Individual chimneys are more numerous than at Santa Eulalia, although they dont 'throw off' so many long, clearly defined mantos and branches (Prescott, 1926). The mineralisation in both the mantos and chimneys is composed predominantly of galena, sphalerite, pyrite and arsenopyrite, with silver and some gold in the primary zone. A little chalcopyrite is found at depth. Acanthite occurs with the galena, with associated barite, calcite and fluorite. Calc-silicates, mainly garnet, hedenbergite and vesuvianite increase with depth, but may be related to pre-mineralisation intrusives (Megaw, et al., 1988).

The orebodies are numerous and diverse in their occurrence. Examples of orebodies are as follows:

Ojuela - Paloma Chimney - which cuts through near horizontal beds at a high angle. it has been followed to a vertical depth of near 1000 m without declining in grade or size. The chimney exhibits great irregularities in the form of branching, splitting and re-uniting, although the total plan area remains relatively constant. In section the chimney has a width of around 40 m (Prescott, 1926).

San Jorge - San Juan Chimney and Manto - starts at its deepest development as a chimney following a fault which has around 100 m of displacement. The fault is vertical with a relatively straight strike, and at many points is filled with post-ore dykes. Approximately 200 m above the water table the chimney becomes a manto and follows the intersection of a limestone bedding and the fault structure to the south for about 500 m, before leaving the fissure in a wall to one side. Soon after this it rejoins the fissure and becomes a chimney again, until it intersects the current surface. The total length of the chimneys and manto is of the order of 800 m, with a cross sectional diameter of the order of 20 m (Prescott, 1926).

Cumbres Manto and Chimney - This orebody lies along the axis of a sharp anticline. Initially it occurs as a group of inclined mantos, each of which is an independent orebody with its own metal ratio. Where the manto swarm encounters a particular pre-mineral fault oblique to its trend, it branches and becomes a chimney system, following the fault for around 100 m before becoming a manto again, then proceeding to the south along the anticlinal axis, over a distance of near 1 km, while diminishing in diameter. In this example the mantos are restricted to the axial zones of an anticline. The total length of the orebody is around 1500 m, but has a cross sectional diameter of around 20 m in the centre (Prescott, 1926).

In other orebodies the mineralisation may follow individual steeply dipping beds as a sheet like body, rather than the finger like form that most mantos assume. It is apparently difficult to differentiate the mantos from chimneys on the basis of either mineralogy or mineral assemblage. They are taken to be part of the same body, although they are always inclined upwards and decline in cross sectional area in the same direction. With depth, the orebodies almost invariably approach single chimneys in form, losing much of the irregularity that marked their upper levels. The fractures that control the chimneys are, in some cases, post-mineralisation fractures. It is suggested that the mineralised structures and trends follow dislocation in the basement which control the folding and faulting in the overlying carbonate sequence (Prescott, 1926).

The most recent source geological information used to prepare this summary was dated: 1988.    
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
   Selected References:
Megaw, P.K.M., Ruiz, J. and Titley, S.R.,  1988 - High-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of Northern Mexico: in    Econ. Geol.   v.83, pp. 1856-1885.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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