Download geological setting of the vms project

GEOLOGICAL SETTING OF THE VMS PROJECT
Most of the known volcanogenic massive sulfide districts in Mexico are hosted in the
Guerrero Terrane, which consists of mostly submarine volcanic and volcaniclastic rocks
of Jurassic and Cretaceous age, interbedded with shallow marine limestones and
siliciclastic sedimentary rocks. These rocks were deposited unconformably on a
Permo(?)-Triassic to Early Jurassic basement dominated by deformed deep marine
siliciclastic sequences (e.g. Campa and Coney, 1983; Centeno-Garcia, et al., 1993;
Centeno-García et al., 2003). Tectonically the Guerrero terrane is interpreted as a
Mesozoic arc and back-arc system (e.g. Freydier et al., 1997, 2000; Dickinson and
Lawton, 2001) that was accreted to the North American continent late in the Early
Cretaceous (Dickinson and Lawton, 2001). Large volumes of calc-alkaline magmas
intruded the Guerrero Terrane after accretion to the Continent in the Late Cretaceous to
Eocene (Schaaf et al. 1995). Covering the Mesozoic rocks are Tertiary to recent volcanic
rocks of the Sierra Madre Occidental, Sierra Madre del Sur and the Trans-Mexican
Volcanic Belt (Morán-Zenteno et al., 1999).
Fig. 1 Map showing windows of the Guerrero Terrane and volcanogenic massive sulfide
deposits in southern Mexico (from Bissig et al., 2006).
ZIM’s Property overlaps most of the window of Guerrero Terrane rocks exposed
southeast of Puerto Vallarta (Fig. 1). In this window, basement schists of the Guerrero
Terrane are mainly exposed west and north of Cuale (Fig. 2). Island-arc rocks are
represented by the Middle Jurassic Cuale Sequence (Bissig, 2006). Regionally, the Cuale
Sequence trends east-northeast, dips gently to the south, and is best exposed between
Bramador and Talpa de Allende. From the base upwards, the Cuale Sequence consists of:
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Quartz-feldspar porphyritic rhyolite pyroclastics and flows
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Aphyric rhyolite pyroclastics and flows
Hornblende and feldspar megacrystic dacite flows and volcaniclastics
Black argillite and sandstone intercalated with all of the above the volcanic units
Late Jurassic-Early Cretaceous aphyric andesite flows.
Fig. 2 Geological Map of the Property. Map compiled from Bissig et al. (2006), the
MMAJ-CRM geological survey of 1984-86 and the author’s own mapping and
interpretation of ZIM’s 2006 geophysical data.
Unit 50: Schist
Pelitic schist, intercalated with chloritic and sericitic schist and meta-arkose is exposed
west of Cuale and Bramador. This package is gently folded and metamorphosed to
subgreenschist facies assemblages. Cleavages are generally oriented west-northwest, and
dip moderately northeast. The base of the unit is not exposed, but the overall thickness is
thought to be at least 800 m (Berrocal and Querol, 1991). It is separated from the
overlying volcanic rocks by an angular unconformity and is locally intruded by
hypabyssal rhyolite, andesitic dikes and granodiorite. Although the age of these pelitic
schists is not directly constrained, a minimum Early to Middle Jurassic age is given by
the U-Pb dates obtained for the overlying Cuale Volcanic Sequence (Bissig et al. 2006).
However, the pelitic schists may well be as old as late Paleozoic to Triassic and can, on
the basis of similar lithology and stratigraphic context, be correlated with the Arteaga
complex 300 km to the southeast (Centeno-Garcia et al., 1993).
Unit 1A: Black shales
Thinly laminated black argillites (1A) unconformably overlie Unit 50 and are intercalated
with Units 10C, 10A and 10D, as well as minor rafts of fossiliferous limestone (Unit 1E).
Most of the shales are not particularly graphitic, but thinly laminated sulfides occur
throughout Unit 1A. Black argillite is not that common at Cuale, where it mainly occurs
in a restricted area between Naricero and Socorredora. On the other hand, black argillite
is the dominant member of the Cuale Sequence at Bramador, and outcrops on the road
between Concepcion de Bramador (La Concha), Bramador and Caballos de Cabrel, in the
Rio Desmoronado and in the arroyo San Jeronimo. In alteration zones, the argillites are
easily altered to pale grey microcrystalline (“cherty”) quartz. Such rocks were probably
mistaken for aphyric rhyolites by some mappers. Similarly, deep weathering of silicified
black shales produces a white, powdery soil that can be mistaken for weathered rhyolite.
Geologic maps by the SGM (e.g. 1:50 000 sheet D13A71) show large tracts of
“Cretaceous rhyolite” north of Descubriadora and south of Aranjuez that are thought by
the author to be Unit 1A sediments of the Late Jurassic Cuale Sequence.
Fig.3 Thinly laminated black shales exposed west of Socorro (looking west). Regionally,
the rocks of the Cuale Sequence strike east-northeast, and dip gently to the south.
Unit 10C: Quartz-feldspar porphyritic Ignimbrite And Cryptodomes
Unit 10C rhyolite is widespread, and is characterized by 2-5% 1-3 mm quartz
phenocrysts, 10-35% feldspar phenocrysts 1-4 mm long, and perhaps 1% mafic
phenocrysts (Fig. 7.5). The unit is a rather typical ignimbrite sequence with: (i) dense
crystal tuff at the base (10CF) overlain by, (ii) welded lapilli tuff (10CFX) that is intruded
and overlain by, (iii) co-genetic coherent flows and cryptodomes (10C) with associated
clastic deposits (10CF). Unit 10C ranges from a few 10’s of meters thick where
sedimentary units dominate to over 400 m thick in the vicinity of some of the larger
domes. Crystal tuff occurs in the footwall below La Concha, and thick units of welded
lapilli tuff are important west of Cerro Caracol and north of Cerro El Canton. The largest
known domes or subvolcanic intrusions occur west of El Coatante (Cabrel Property) and
southeast of San Juan. Flow-domes commonly exhibit flow-banding, spherulitic
devitrification, amygdules and lithophysae and are commonly enveloped by large
volumes of monomict hyaloclastite breccia. At Cuale, some breccias are re-worked to
conglomerates (1D), suggesting locally emergent conditions. At Bramador,
conglomerates are absent in Unit 10C.
Uranium-lead radiometric dating of Unit 10C lapilli tuff from the hangingwall of the La
Prieta Fault at Cuale yields an age of 159.2 +/- 2.2 Ma for the first ignimbrite eruption.
Two results for coherent flow units are 157.2 +/- 0.5 Ma and 152.5 +/- 1.5 Ma (Bissig et.
al, 2006). The dates make geological sense as the ignimbrite blast would typically
precede the emplacement of flows and domes.
Volcanogenic massive sulfides occur in clastic members of Unit 10C that are intercalated
with black argillite. Most of the historic workings in the arroyo San Jeronimo near
Bramador and massive sulfides in La Concha are closely associated with crystal tuffs of
Unit 10C. At Cuale, the San Nicolas deposit is hosted in Unit 10C rhyolite.
Fig. 4 Quartz-feldspar porphyritic dome exposed west of the Socorro Plant (El Coatante
area).
Unit 10A: Aphyric Rhyolites
Aphyric rhyolites are characterized by 1% <1 mm quartz phenocrysts and rare feldspar
phenocrysts. Overall, the unit represents a second ignimbrite blast with (i) welded lapilli
tuff (10AX) at the base and, (ii) co-genetic flow domes (10A), and fine tuffs intercalated
with black argillite (1A). Below Jesús María, tuffaceous rocks of Unit 10A contain
accretionary lapilli (Bissig et al., 2006). Accretionary lapilli form in the ash cloud, but
can be preserved in subaqueous sequences if they are buried quickly. Unit 10A is on the
order of 10 to 100 meters thick. A U-Pb date of 154.0 +/- 0.9 Ma for aphyric rhyolite
from Cuale is reported in Bissig (2006). Unit 10 A is not that well exposed in the Cuale
Sequence, yet is it closely spatially related to ore both at Bramador and at Socorredora
and Naricero.
Fig. 5 Flow-banded aphyric rhyolite flow-dome near junction of Rio Desmoronado and
the arroyo San Jeronimo (Bramador). The blobular areas between the flow-bands are
spherulitic.
Fig.6 Aphyric rhyolite fragments in black argillite matrix from the footwall to
Socorredora, Cuale mine site.
Unit 10D: Feldspar and Hornblende phyric dacite
A third pulse of supracrustal felsic volcanism is marked by feldspar-phyric dacite crystal
tuffs, flows and breccias. The characteristic phenocryst assemblage for Unit 10D is 30%
10-30 mm long feldspar megacrysts and 3-5% hornblende phenocrysts hosted in a pale
grey, siliceous (almost cherty) matrix. Quartz phenocrysts are rare or absent. The base of
the Unit is composed of crystal tuff (10DFX) intercalated with layers of black argillite
and grey limestone (Unit 1E). In some areas, the bedding is smooth and rhythmic, in
others it is chaotic with rip-up fragments of black argillite supported in a matrix of crystal
tuff. Crystal tuffs form by separation of low density glassy shards into the eruption
column from the higher density crystals which are sedimented downhill from the vent.
The hot turbulent suspensions mix with sea water and sediments on the sea floor to form
steam-inflated density slurries (e.g. Fig 7.8).
Flow facies of Unit 10D are pillowed with inter-pillow black argillite sediments (Fig.
7.9), and the margins of the flows grade into monomict hyaloclastite breccias. In some
areas, individual flows of Unit 10D are very thick, and the supracrustal characteristics are
not well observed. This led to the unit being mapped as a younger “intermediate
intrusive”, first by the MMAJ and later by the SGM (e.g. geological map for F13 D71).
The field evidence from Bramador shows unequivocally, however, that Unit 10D
represents a dacitic seamount, which may or may not have some intrusive components.
Overall, Unit 10 ranges in thickness from about 100 meters to 1200 meters thick.
Fig. 7 Feldspar-phyric crystal tuff (Unit 10DF) with blobular rip-up fragments of black
argillite.
Fig.8 Feldspar megacrystic and hornblende phyric dacite (10D) pillows with black
argillite (1A) between pillows. These pillows are characteristic of underwater marine
eruptions.
Unit 1C: Sandstones and Fine Conglomerate
The topography of the Cuale Sequence would have been irregular, and some of the
volcanic edifices were locally emergent, particularly near Cuale. Evidence for emergent
conditions include abundant but local conglomerate (stream lag) deposits, accretionary
lapilli and minor reef limestone deposits intercalated throughout the volcanosedimentary
pie. The MMAJ mapped a significant sandstone horizon south and west of Aranjuez.
Unit 11A: Andesite
Andesitic flows and volcaniclastic rocks outcrop on the ridges of Cerro Trinidad (west of
Arroyo de la Mina). Rocks are medium green, propylitized and plagioclase-pylotaxitic.
Although contact relations between Unit 10D and Unit 11A were not observed during
this campaign, the contact is believed to be conformable based on the absence of
conglomerates between the Units north of La Parota. At Cuale, four subvertical, WNW
striking andesitic dikes cross-cut the intrusive rhyolites (Bissig, 2006). These dikes are
probably related to the supracrustal andesites on Cerro Trinidad.
Fig. 9. Andesitic tuffs south of Arroyo de la Mina.
Unit 21 B: Granodiorite
Finally, the youngest rock unit found in the study area is massive and unaltered
granodiorite (unit 21B) intruding the Cuale Volcanic Sequence. It is assigned to the
Middle to Late Cretaceous Puerto Vallarta Batholith, which, in the vicinity of the Cuale
district exhibits weak peraluminous characteristics (Schaaf et al., 1995).
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