Summary:
The Bilbao deposit is characterised by skarn minerals (eg. hedenbergite (pyroxene), wollastonite and other calcsilicates) and can be classified as a polymetallic skarn deposit.
Base metal skarn-type deposits commonly form a continuum with Carbonate Replacement Deposits (=CRD‘) that are generally developed distally from intrusions. In Mexico such deposits are known as manto type deposits when the mineralization forms a coherent massive sulphide sheet.
The Bilbao mineralization is classic contact metamorphic skarn type developed by irregular replacement of carbonate. However, there are marked constraints on this irregularity in that the mineralization selectively replaces certain horizons in the stratigraphy of the limestone and so assumes lenticular form. Furthermore, there is a strong tendency for mineralization to occur at the granitic contact. Because the mineralization is related to fluids emanating from the (hot) granite this impinges on the mineralogical zoning away from the heat source and results in higher temperature minerals being deposited closer to the main granite contact. This has implications on primary mineralogy, especially for zinc, which in this case appears to have formed several mineral species in congruence with the above zonation. Thus at the higher temperatures prevailing close to the granite franklinite, willemite and hemimorphite have developed whereas sphalerite appears to have formed further away from the contact.
The recognition and delineation of oxide and sulphide mineralization is of importance in relation to their metallurgical recoveries and to the overall economics of any future mining operation at Bilbao. It is therefore important to classify and segregate resources according to mineralization type.
Discrimination of sulphide, mixed and oxide mineralization for the purposes of resource estimation has been carried out principally by examination of descriptions of the narrative logs. Mineral zones have then been classified as sulphide, oxide and mixed according to the predominance of ore types within the zone. Within the central core of the deposit sulphide and oxide zones are thus depicted on section as separated by a mixed zone up to 60 metres thick.
Sulphide Mineralization
The principal sulphide minerals present in the primary mineralization are pyrite, sphalerite, galena and chalcopyrite. Examination of these sulphides in drill core indicates that they occur mostly as separate grains frequently from 1-5mm in size, and can be easily recognized. The sphalerite occurs as the iron-rich black jack variety, which is relatively easy to recognize in comparison to pale or fine-grained varieties that have not been recognized at Bilbao. Minor or rare sulphide minerals recognized in the Bilbao core include pyrrhotite, marcasite, arsenopyrite, bismuthinite, bornite, molybdenite and guanajuatite.
Accompanying these sulphides are the primary oxides franklinite and minor cassiterite. Furthermore willemite, the zinc silicate, (and rare larsenite its Pb analogue) complete the skarnoid mineralogical suite. The gangue hosting these sulphides comprises limestone, or more usually calc-silicate rich marbleized limestones.
The presence of minor tin and tungsten values indicates a granitic affinity and an association with other Mexican manto-type deposits.
Sulphide mineralization has been recorded at a minimum depth of 58 metres and to a maximum depth of 298 metres below surface.
Oxide Mineralization
Most mineralization within 100 metres of surface has been completely oxidized by reaction with meteoric waters. The oxide part of the body is directly derived from weathering of the primary sulphides, and because of this the grades are more or less the same as those occurring in the sulphides.
Historically, the oxide material was mined in the open pits and underground with zinc and copper recovered by leaching with dilute sulphuric acid or directly shipped to the plant to recover all metals. (Note: historical production was shipped directly to a smelter in Texas and not treated on site).
The oxidized fraction of the deposit has an altered mineralogy compared to that in the primary mineralization in that silicates, sulphates, phosphates and carbonates predominate. The bulk of the gangue in the oxide ore comprises a mixture of quartz, iron-oxides/hydroxides, carbonates, phosphates, sulphates, arsenates and silicates. Through scanning electron microscopy (SEM) work undertaken by the University of San Luis Potosí and mineralogy undertaken by Grammatikopoulos et al (2008) of SGS, the mineralogical composition of the Bilbao oxide is much better known than when originally studied by Kilborn. The main points emerging from these studies of the mineralogy are listed below:
1. The composition of the whole is made up chiefly of iron oxides and quartz with about 7% metals principally lead, zinc and copper together with roughly 3 ounces of silver.
2. The mineralogy of the gangue is governed by two factors. Firstly, it is derived from a primary contact skarn and secondly it formed from oxidation of a primary pyrite-rich sulphide body; the whole being weathered and altered by secondary acidic reactions generated by decomposition of pyrite into dilute sulfuric acid solutions by meteoric water. This has resulted in resistate minerals such as franklinite, cassiterite, willemite, pyroxenes, and quartz being intermixed with secondary alteration products such as oxides, hydroxides, carbonates, sulfates, phosphates, arsenates and clays. Complicating the issue is that, in part, the oxidation is incomplete so that some remnant sulphides persist in the oxide part; this is especially so with respect to sphalerite.
3. The gangue minerals identified in the mineralogical study are:
I. Fe-oxides: hematite, goethite Clays: nacrite, kaolinite.
II. Sulphates: anhydrite, barite, gypsum, basaluminite,
III. Carbonates: calcite
IV. Arsenates: ogdensburgite
V. Silicates: pyroxene, quartz and aerinite
4. Lead occurs in the oxide, predominantly as the unusual mineral hedyphane Ca2Pb3(AsO4)3Cl. Formerly this was considered a type of mimetite - the yellow lead mineral found on the dumps at Bilbao, but is now classified as an end-member of the apatite group along with phosphohedyphane)
5. Minor amounts of galena, lead oxides and lead/ manganese-oxides are also found in the oxide.
6. Trace amounts of other lead minerals such as phosphohedyphane Ca2(Pb,Ca)3(PO4)3Cl and corkite PbFe3+(PO4)(SO4)(OH)6 also occur.
7. Zinc in the oxide fraction occurs as sphalerite, willemite and hemimorphite.
Oxide mineralization has been recorded to a maximum depth of 215 metres from surface (in hole X20). Most mineralization above 100 metres depth (2050m elevation) is completely oxidised, with the degree of oxidation diminishing with depth so that mineralization below 150 metres (2000m elevation) is largely unaffected by oxidation. The distribution of oxidation appears to be controlled by the permeability of the rocks, and is controlled largely by faults and other fractures. Oxidation has preferentially affected the outer surfaces of the sulphide lenses, so that the thinner lenses are completely oxidised, whereas thicker lenses retain a sulphide core surrounded on the margins by an oxide shell.
The interface between oxide and sulphide mineralization is not always sharp and easily defined. Some intersections comprise many metres of partly oxidized material within which sulphide remnants occurring as lenses or patches from a few centimetres up to several metres wide may alternate with oxidized or partly oxidized material to form mixed mineralization.
Mixed mineralization has been recorded at a minimum depth of 28 metres and to a maximum depth of 229 metres below surface.