The Metates gold-silver-lead deposit is located 160 km
northwest of Durango in the Durango State in Mexico. The deposit is located
within the Sierra Marde Occidental. Exploration in the area began in the late
1970s, likely delayed due to the remote and rugged nature of the landscape.
Regional geology is poorly described with much of the Sierra
Marde Occidental underlain by Tertiary volcanics. The older Lower Sequence
consists of andesite flows and breccias, while the younger Upper Sequence
consists of predominantly rhyolite ash flow tuffs and ignimbrites. A
conglomeritic unit of variable thickness (up to 60m) is known to underlie the
lower sequence and may be an erosional accumulation of the Mesozoic
conglomerate. It appears to be locally enriched in silver secondary
mineralisation. The lower sequence consists of andesite flows up to 100 to 150m
thick while the breccias have experienced propyllitic alteration. The upper
volcanic sequence is up to 700m thick in the project area and consists of
rhyolite ash flow tuffs that form prominent cliffs in the area. The Tertiary
volcanic sequence appears to post date mineralisation and hosts no economic mineralisation.
Locally the flat-lying Tertiary volcanic cover has been
eroded to expose the Metates deposit, hosted in Mesozoic basement rocks. This
basement complex consists of monotonous sandstone, shales and argillites of
Cretaceous to Jurassic age. These sediments have been isoclinally folded in
places and duplex structures may be present making the section appear thicker
than it actually is (1,000 m). The lack of marker beds compounds the difficulty
in determining the sequences thickness. Towards the base the sequence is
dominated by thinly-bedded fine grained sediments with grain size generally
The sequence is variable enriched in black carbonaceous
material with organic carbon ranging up to more than 1%. Pyrite generally
ranges from 5 to more than 10% and is present as thin laminations and as
disseminated biogenic framboidal pyrite, as well as irregular veinlets or
stockworks. The stratigraphy indicates a deep marine, seafloor, distal flysch
A 100m thick section of rounded pebble and boulder
conglomerate is preserved in the Upper Mesozoic sedimentary sequence. Clasts
are sandstone and subordinate shale, chert, volcanic and quartz fragments in a
well-indurated sandy shale matrix. Arkosic and argillitic interbeds are common.
The Metates Intrusion is an inverted saucer-shaped body
interpreted as a subvolcanic to extrusive volcanic dome. It is oriented NW-SE,
dips 40o to the NE and has a strike length of 1.5 km and is up to
300m thick. The quartz latite body contains at least 50% quartz, biotite and
feldspar phenocrysts in an aphanitic groundmass. Pyrite content is similar to
the surrounding sediments (5-10%). The upper contact or transition is generally
conformable and is primarily made up of an igneous breccia with rounded igneous
clasts in an igneous matrix that transitions up section to a sedimentary
dominated breccia up to 100m in thickness. U/Pb dating of zircons from the
Metates Intrusives gave an emplacement age of 108 Ma. Sericite K/Ar ages of 87
and 89 Ma have been interpreted as alteration ages.
Phyllic alteration has extensively affected the Metates
Intrusion with feldspars being altered to sericite and biotite altered to
pyrite. Fine grained sericite and quartz have replaced virtually the entire
groundmass. Argyllic alteration is poorly developed in the intrusive and
silicification is rare and patchy. Weak propylitic alteration is present in the
sediments but these are generally unreactive due to their fine grain size. The
intrusive is interpreted to have been emplaced at low temperature due to the
lack of evidence of contact metamorphism in the enclosing sediments.
There is evidence of both syngenetic and epigenetic sulphide
mineralisation at Metates. Syngenetic pyrite mineralisation, typical of euxinic
(black shale) environments, is disseminated throughout the sediments. Little if
any precious metal mineralisation is attributed to this stage. Epigenetic
mineralisation appears to have occurred in two phases, one in the sediments
(North Zone), perhaps a distal event related to an intrusive dome, and another
within the intrusive (South Zone).
Mineralisation occurs as sulphide stockwork veinlets 1 to
5mm thick (up to 1cm) and disseminations consisting of pyrite, sphalerite,
arsenopyrite and galena. Quartz and calcite gangue is uncommon. Veinlets are
commonly banded with pyrite, sphalerite and/or galena layers. Pyrite and
sphalerite replace feldspars and biotite within the intrusive and sphalerite
and galena inclusions are often found within veinlet and disseminated pyrite.
Gold mineralisation occurs as microscopic native gold and
electrum, usually included within pyrite, as exsolutions within pyrite or
included within the crystal structure of pyrite, both in the sediments and in
the intrusive. Gold is refractory and therefore not suitable for regular
cyanidation, even when finely ground. Silver is also refractory, although to a
lesser extent than gold, occurring within galena and as argentite. Gold and
silver occur with sulphides replacing phenocrysts and in veinlets and
stockworks. The significant carbon content of the sediments also provides a
metallurgical challenge due to its characteristic “preg-robbing”, where
cyanide-liberated gold and silver are bound up with organic carbon.
The textural relationships observed at Metates indicate
multiple mineralising events. Base of oxidation is quite shallow over the
deposit (5 to 10m deep) with fresh sulphides observed on the surface.
NI 43-101 compliant Reserve as of March 2013: Total
proven/probable reserve of 18,452 koz Au @ 0.5 g/t, 526,111 koz Ag @ 14.2 g/t,
4,185 Mlbs Zn at 0.17% Zn.
Austin, D.C. et al, 2013. Metates Gold-Silver Project: NI
43-101 Technical Report Preliminary Feasibility Study prepared for Chesapeake
Gold Corp. M3 Engineering & Technology corporation.