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Possible Pachuca-type epithermal system. Pachuca-Real del Monte is the second largest epithermal deposit in Mexico with total production of roughly 80 M tonnes with an average grade of 500 g/t Ag, and 2.5 g/t Au.
The 7,256 hectare Pachuca SE claim is 24km SE of the city of Pachuca, Hidalgo (Figure 1). Infrastructure is excellent, with power availableand easy road access from the Pachuca-Tulancingo freeway which crosses the middle of the property.
Pachuca SE is on strike from one of the world's largest epithermal precious metal deposits and the local geology is typical of a high-level epithermal environment.Epithermal-style argillic alteration is associated with anomalous Au, Ag and Zn geochemistry, exposed in an erosional window into a felsic volcanic pile. The anomalous geochemistry and clay alteration are essentially identical to that seen in the Vizcaina structure above the Pachuca Mine.
The Pachuca SE target has size, hydrothermal alteration and geochemical anomalies associated with structures and could host a preserved Pachuca-type epithermal deposit at depth.
The Pachuca SE system occurs in a 5km x2km NW-SE elongated erosional window. It was found during regional prospecting and mapping of the historic Pachuca mining district. Prospero's geologists investigated a number of small pits that were being mined for industrial clay and noted the presence of epithermal style smectite-illite alteration associated with some major fault structures. At higher elevations the alteration appears to be high-level silica-kaolin-alunite alteration (e.g. Cerro El Varal.)
This important identification of previously unrecognised epithermal type alteration so close to the Pachuca mining camp, prompted Prospero's team to undertake detailed mapping and geochmical sampling of exposures of alteration in road cuts which are known to be 100-200m vertically above some of the highest grade ore bodies of Vizcaina vein in the historic Real del Monte mine.
The work has shown that to all intents and purposes, the Pachuca SE alteration is identical in terms of clay mineralogy and anomalous Au-Ag-Zn geochemistry, suggesting potential for a blind Pachuca-type orebody below the Pachuca SE pits. No modern day exploration has been conducted on Pachuca SE.
A number of small adits and shafts were developed along structures at Pachuca SE, presumably by the Pachuca-Real del Monte operators some hundred years ago. They excavated several levels over a 100m in length and shafts over 30m deep along the Pachuca SE structures, most likely expecting to find the tops of orebodies but failing to do so.
The project sitson the eastern margins of the Sierra Madre Oriental in a highly mineralized NW-SE trending belt that includes three important historic producers of precious metals: Pachuca-Real del Monte; Zimapan and La Negra. Southeast of Pachuca the Tertiary volcanic belt is covered by younger rocks which also cover the related hydrothermal systems.
The historic Pachuca mining district consists of a largely preserved conjugate vein system oriented E-W and N-S, hosted within the Pachuca Group andesitic volcanic sequence. At the Real del Monte mine some orebodies were exposed in valleys but most of the veins and orebodies were blind and were found as the deposit was mined underground.
Along the same trend 120km southeast of Pachuca, Almaden Minerals has discovered the Ixtaca project. They sampled a small outcrop with anomalous silver and gold values which is located 100m under a barren silica-argillic alteration cap.
The rock units in the area are dacites of the Pachuca Group, which are unconformably overlain by rhyolites. Both units are intruded by dacite and rhyolite dikes. This volcanic stratigraphy is buried by surrounding andesites and basalts of Quaternary age (the Neovolcanic sequence) (Figure 2).
Hydrothermal Alteration and Structures
Two styles of hydrothermal alteration have been recognised by Prospero at Pachuca SE. The alteration zones are developed in permeable rock units adjacent to structures. The mapped structures can be 600m to 1,200m long although they may be longer -- to the east and west they disappear under younger volcanic rocks (Figure 2). Outcrops are scarce and are largely observed in the clay open pits or in some of the historic workings, but the general trace of the structures can be deduced from structural and stratigraphic mapping.
At higher elevations in the southern zone, silica-kaolin-alunite alteration occurs at Cerro El Varal. The alteration is stronger where structures are present producing bleached and microcrystalline silicified reefs, with silicification weakening into bleached rhyolite laterally. The silicified reef -which is up to 15m to 20m wide- appears to be associated with normal faulting that had a sinistral component to its movement.
The larger alteration zones are observed associated with E-W oriented zones, following the trace of the main structures, along the margins of the rhyolite dike in Cerro Varal, or along dacitic dikes. On the southern flank of Cerro Varal a group of multiple narrow veins occur within an E-W trend 900m long, consisting of banded silica and chalcedony with locally abundant pyrite, and silicification of the intrusive host rock.
At lower elevations in the more northern and central parts of the project, smectite-illite alteration was mapped with oxidized pyrite, suggestingdeeper geologic levels. This alteration is currently being mined from a series of small pits for industrial clays. In dikes and dacite host rocks of the northern sector the hydothermal alteration is white, whereas in the andesites it is greenish.
Fluid Inclusion Petrography
In the northern sector, fluid inclusion work onsmall quartz veinlets showed temperatures of homogenization between 230°C to 270°C. In the Pachuca-Real del Monte region this same range of temperatures is present 150m to 200m above the top of economic ore shoots in the higher level argillic alteration zones of the Vizcaina and Purísima veins. No measurable fluid inclusions were found in the higher-elevation southern Cerro Varal area, since the reefs are either silicified host rocks or free silica that is cryptocrystalline.
The zoning of alteration suggests a deeper geologic level is exposed in the north and middle sectors of the project area, whereas the top of the system with acid-sulfate alteration occurs along the crest of Cerro Varal (Figure 3), indicating proximity to a paleo-water table and the top of the system.
Prospero has compared the alteration exposed above known ore bodies at Pachuca to the alteration seen in the pits at Pachuca SE. Above knownbut blind veins the geology is characterized by faults with minor silica and calcite veining, and argillic alteration caps.
Eleven samples were collected from argillic alteration in road cuts exposing the Vizcaina fault which is 100m to 200m above the top of known orebodies. Ag and Au assays were below detection limit (0.02 ppm Ag; 0.002 ppm Au), with weak Zn (100-200 ppm), and Ba (<250 ppm) anomalies.
It is remarkable that one of the largest epithermal deposits in the world (total production of 1.4B ounces Ag, and 7M oz Au) sits beneath such low-level geochemistry: the geochemistry is non-diagnostic.
The character of the faults and argillic alteration at Pachuca SE, located 20km southeast and nearly along strike of the southeast end of the Vizcaina vein, is similar to the high level geology above the orebodies at Pachuca-Real del Monte.
134 rock-chip samples from Pachuca SE were assayed with lower limits of detection of 1 ppb for Au and 10 ppb for Ag. The highest precious metal and zinc geochemistry occurs in the northern sector with anomalous gold assays in the 0.1 to 0.61 ppm range, silver assays in the 1.0 to 2.92 ppm range, and zinc assays of 100 to 5,290 ppm (Figure 4, 5, 6). These values coincide with the presence of narrow coarse crystalline quartz and calcite veins and iron oxides closely associated with the mapped structures.
In the Cerro Varal area the intrusive rhyolite hosts numerous pyrite-rich veins along an exposed length of 900m. The veins consist of cryptocrystalline quartz carrying anomalous values of Ba (up to 3780 ppm; Figure 7), Mo (up to 561 ppm; Figure 8), and As (up to 209 ppm), exhibit dickite alteration in the rhyolite intrusive wall rocks, and occur immediately below the cap of advanced argillic alteration (alunite-kaolinite). Numerous workings and shafts up to 30m deep were developed by the old timers along the pyrite-silica rich veins.
Comparison With High Level Alteration at Pachuca
The surface expression of Pachuca SE is similar to the high-level epithermal environment that overlies the historic Pachuca Mine veins. In fact, as mentioned above, the old timers dug adits and shafts at Pachuca SE, probably over a hundred years ago, most likely expecting to find the top of Pachuca-type orebodies.
A geochemical comparison of the maximum values obtained for precious metals, base metals, and pathfinder elements from the high-level Vizcaina structure at Pachuca and at Prospero's Pachuca SE is shown below:
Pachuca SE North Sector
Pachuca SE South Sector
Vizcaina high level
Consistently higher geochemical anomalies are seen in the Pachuca SE North sector when compared to both the Pachuca SE South sector as well the Vizcaina high level, indicating that the present level of erosion in the argillic cap in the north sector could be closer to the top of possible mineralization.
Similarly, the Pachuca SE South sector exhibits higher geochemistry than the Vizcaina high level, which is supportive that this sector could potentially overlie a mineralized zone at depth.
Pachuca SE is located within the sequence of volcanic rocks that hosts the historic Pachuca-Real del Monte mining district. The Pachuca Mine represents the second largest epithermal deposit in Mexico with total production estimated in the order of 80M tons with an average grade of 500 g/t Ag, and 2.5 g/t Au (Albinson et. al., 2001: SEG Special Publication Series No. 8, p. 1-32).
The Pachuca SE geology appears to be typical of a high-level epithermal environment with the following characteristics. The project is located along strike from one of the world's largest epithermal precious metal deposits. Sampling has shown attractive Au and Ag geochemistry when compared to the geochemical signatures of the Vizcaina structure which sits directly above the Pachuca Mine. The clay alteration assemblage in the argillic alteration caps of both systems is similar.
Pachuca SE is also associated with a more felsic volcanic pile than the Pachuca-Real del Monte host rocks. Many of the most important epithermal precious metal deposits in Mexico are closely time-related with emplacement of domes of felsic composition, or include significant volumes of felsic volcanic rocks in the pre-mineral phases of volcanism (rhyolites to dacites). Examples include:
In summary, the Pachuca SE target has size, hydrothermal alteration and geochemical anomalism associated with structures. In many aspects it is similar to the known high-level alteration of the Pachuca-Real del Monte district, and could host a preserved epithermal deposit at depth.