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Santo Tomas |
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Sinaloa, Mexico |
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Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Santo Tomás Cu‐(Mo‐Au‐Ag) porphyry copper deposit comprises three zones, South, North and Brasiles, and is developed within the western margin of the Sierra Madre Occidental in the state of Sinaloa, crossing the boder into neighbouring Chihuahua, in western Mexico (#Location: 26° 53' 19"N, 108° 11' 34"W).
Artisanal miners have sporadically worked both the North and South Zones at Santo Tomás since the early 1900s, havin dug several small excavations and two small adits. Between 1968 and 1971, ASARCO drilled 43 vertical diamond holes for 13 697 m of core and 16 vertical rotary percussion holes (1391 m), mostly in the North Zone, before exited the Property in 1973. In the same year, Tormex Mining Developers Ltd. and Industria Minera Peñoles optioned the Property and conducted exploration and re‐sampling to 1977, mainly on the North Zone, as well as relogging the ASARCO core and drilling 2401 m in 7 new diamond drill holes, before producing a resource estimate. In the 1980s and 1990s, the deposit fell within a series of regional airborne magnetic surveys, helicopter-borne geophysical surveys, LANDSAT imagery and geological mapping by Mexican government agencies. In 1990-91 the Esmeralda Group and Minera Real de Ángeles S.A. de C.V. interpreted existing data and produced revised mineral resource calculations. In 1992 Cerro de Cobre Inc. entered into a purchase agreement with the Esmeralda Group for the Property and then optioned it to Exall Resources Ltd., who focused on the higher-grade near‐surface oxide zone. In 1993, Exall conducted a 4000 m drill program comprising 33 reverse circulation and 7 diamond drill holes, and produced a new resource estimate. A pre-feasibility and engineering study and metallurgical testing were undertaken by Bateman Engineering in 1994. Between 1992 to 1995, the Luis Donaldo Colosio Dam ('Huites Dam') was constructed 15 km downstream from the property on Río Fuerte, raising the maximum water level by ~70 m in the reservoir to impinge upon the northern flank of the Santo Tomás deposit. In 1997, Exall relinquished its option on the property. Morgain Minerals Inc. and its wholly owned Mexican subsidiary Minera MGM S.A. de C.V. entered into an agreement with Sr. Rubén Rodríguez Villegas (Rodríguez), the title holder, to acquire a 100% interest in the Property and conducted further metallurgical testing between 1997 and 1998. In 2002, Rodríguez transferred 100% ownership of the Property to Compañía Minera Ruero, S.A. de C.V., a private registered company in Mexico owned by Ruero International Ltd., a Bahamas based company which Rodríguez controlled. In 2002, Fierce Investments Ltd., a USA company, entered into a Share Purchase agreement with Rodríguez to acquire the shares of Ruero International. Further programs of re-logging of core, mineral resource estimates, plant design, metallurgical test work and technical reviews were conducted. In 2015, 100% ownership of Ruero International reverted to Rodríguez under a decision of the Supreme Court of the Commonwealth of the Bahamas. In June 2016, Xochipala Gold, a subsidiary of Canadian company Oroco Resource Corp., acquired a 100% interest in the Property from Compañía Minera Ruero. In 2017, Oroco initiated a comprehensive geological mapping program of the entire project area, conducted comprehensive geophysical surveys, a drilling program and re-evaluation of past data. In 2023, Oroco Resources released a Mineral Resource Estimate in an NI 43-101 Technical Report to the TSX.
Regional Setting
Santo Tomás lies within the Tahue Subterrane of the larger Guerrero Composite Terrane, the largest of the allochthonous terranes accreted to the continental margin of North America in the Mesozoic and earliest Cenozoic. This sub-terrane has a Palaeozoic basement comprising an accreted arc and associated clastic sedimentary rocks, overlain by Triassic rift-related meta-igneous rocks. These rocks are unconformably overlain by Mesozoic and Cenozoic volcanic and sedimentary sequences that comprise carbonate‐rich sediments, including limestone, marble bodies, sandstones, and large volumes of Jurassic to Cretaceous andesitic volcanic rocks (Borovic, 2006). The andesite commonly encloses thick limestone, marl and marble beds. All of these rocks are intruded by the Late Cretaceous Sinaloa‐Sonora Batholith, which multiple phases of intrusive rocks ranging from diorite and tonalite to granite and quartz‐monzonite. The emplacement of these intrusions was partially controlled and subsequently offset by several phases of Late Cretaceous to Cenozoic faulting. In the Santo Tomás district, the batholith is mostly granodiorite and tonalite. Locally, 90 to 40 Ma Laramide-age intrusive rocks are emplaced in north and NE trending fault zones and post‐date the Sinaloa‐Sonora Batholith. Such intrusions in the Santo Tomás district are of Paleocene age.
These intrusive events are interpreted to have been related to Laramide contraction during subduction of the Farallon oceanic plate beneath North America. This contraction was followed by a period of Basin and Range extension, opening of the Gulf of California, and deposition of the voluminous, regionally extensive, mid-Cenozoic Sierra Madre Occidental volcano‐sedimentary sheet, one of the largest continuous ignimbrite province in the world (Aguirre-Díaz et al., 2008). In the Santo Tomás district, these comprise a sequence that includes Oligocene sandstone and polymictic conglomerate, overlain by thick andesite and rhyolite tuff units. Dacitic volcanic rocks from this sequence some 15 km to the NE, are dated as Eocene at 59 Ma where they are post-mineral. Younger units comprise large volumes of silicic ash‐flow tuffs and rhyolitic lavas, known as the 'Rhyolite Tuff and Ignimbrite'. Coeval feeder dykes and small felsic intrusions are locally described as 'Rhyolite Intrusions'.
Geology
Porphyry style mineralisation at Santo Tomás is associated with Laramide‐age porphyritic quartz monzonite dykes and minor stocks dated at 57.2 ±1.2 Ma (K‐Ar dating; Bridge, 2020) emplaced into Jurassic‐Cretaceous units that comprise metamorphosed andesite, limestone and minor argillaceous and clastic units.
The local stratigraphy include:
• Jurassic to Cretaceous andesite - an extensive, >500 m thick, undifferentiated andesitic succession, mostly massive flows with poorly developed bedding. The andesites are medium to dark grey, aphanitic to fine-grained, and may be porphyritic, containing 10%, 1 to 3 mm subhedral plagioclase phenocrysts.
• Andesite tuff, a unit differentiated within the regional andesite succession described in the previous point. It overlies more massive andesites and immediately underlies a limestone (see below). The thickness is very variable, with a maximum drilled intersection of 371 m. It is typically fine-grained and medium to dark grey with a greenish tinge. The texture varies from massive and aphanitic to fine-grained and porphyritic, with up to 8 mm plagioclase and locally aligned, scattered, up to 2 mm mafic phenocrysts.
• Limestone, which forms prominent cliff faces and scarps. It is typically massive, fine to coarse‐grained, and frequently recrystallised to marble. The maximum drilled thickness encountered is 352 m. Bedding is poorly defined, although a crude layering may be observed locally.
• Siltstone and conglomerate, including argillaceous, arenaceous, pebbly quartz conglomerate and sandstones defining a clastic sequence that immediately overlies and may be interbedded with the upper sections of the limestone unit. Maximum thicknesses in different parts of the deposit area of 99 to 111 m have been drilled. The siltstone units are characteristically pale brown to greenish, fine to medium grained siltstone-sandstone that may be laminated, with occasional normal graded bedding preserved. Conglomerates are pale greenish grey, mature mainly clast-supported, quartz conglomerates. Clasts are moderately to well sorted, mainly composed of subrounded to rounded quartz that are <2 cm across, with occasional igneous clasts of silicified monzonite and andesite. The unit is generally massive, although graded bedding may be evident locally.
• Sierra Madre Occidental Volcanic Rocks form a discontinuous blanket over the tops and flanks of the ridges at Santo Tomás. Two units are defined locally; i). a lower suite characterised by debris flow (diamictite) deposits and rhyolite breccias, and ii). an upper unit largely comprising rhyolite tuff. The lower diamictite is typically red-hematite stained and matrix-supported. Clasts are polymictic, poorly sorted, angular to rounded, usually <15 cm in size but locally up to 30 cm. Clasts commonly comprise fragments of the underlying limestone with porphyritic rhyolite, andesite, altered monzonite, and quartz, and may be elongated and aligned, and in places are normally graded. The rhyolite breccia of the lower unit is ~145 m thick and overlies the diamictite. It is medium red to pale pink, with a fine-grained matrix containing clasts that are mostly matrix-supported, but locally clast-supported. Clasts are moderately to poorly sorted, and aligned in places, and subangular to rounded, including up to 8 cm rhyolite with subordinate up to 4 cm andesite. The Rhyolite Tuff of the upper unit is a pale to dark pink, fine-grained, crystal-rich, tuffaceous rock, with an ash-lapilli matrix. Crystals are typically <2 mm across and include subhedral to anhedral biotite, amphibole, quartz and plagioclase. It includes common, up to 4 mm, angular to subangular volcanic fragments, mainly of rhyolitic composition, with oxidised fiamme. It is mostly massive, although some intervals may have flow texture and normal graded intervals. The thickest interval drilled is 156.5 m. The Sierra Madre Occidental was deposited on a surface which had significant relief, with erosion exposing the weathered lower succession and Santo Tomás mineralisation although it has served to preserve the mineralisation from post-Sierra Madre Occidental weathering and erosion.
Intrusions in the deposit area include the:
• Late Cretaceous Sinaloa-Sonora Batholith, which at Santo Tomás comprises granodiorite that is mainly exposed to the west of the Santo Tomás mineralisation and extends northward. It is light to medium grey and medium grained, with a phaneritic texture composed of subhedral feldspars, anhedral quartz, dark brown euhedral biotite and euhedral hornblende, disseminated magnetite and traces of pyrite. Some plagioclases crystals are visibly zoned and have a sieve texture.
• Late Cretaceous to Paleocene Laramide Intrusions, including:
- Monzonite to Quartz-Monzonite - with which the Santo Tomás Cu (Mo‐Au‐Ag) porphyry mineralisation is closely associated, is emplaced into Jurassic to Cretaceous andesite and limestone strata. The monzonite phase is typically pinkish pale-grey, varying from a porphyritic texture with a very fine-grained groundmass to a crowded crystal-texture. Phenocrysts include feldspar that are usually 3 mm across but may be up to 9 mm, subhedral to euhedral biotite, amphibole and anhedral quartz. The Quartz-Monzonite is pale-grey and porphyritic with a variable phenocryst content of from 45 to 60%. Feldspar phenocrysts are subhedral to euhedral up to 5 mm across, with sporadic 1.5 cm zoned plagioclase. Mafic phenocrysts include up to 4 mm subhedral to euhedral biotite and amphiboles. Quartz is typically anhedral, and constitutes <10% of the rock. The average modal mineralogy for the Quartz-Monzonite is 35% K feldspar; 8% quartz; 11% mafic minerals, mainly hornblende and biotite; 0.5% sulphides and 2.5% others.
- Hornblende Quartz-Monzonite, which outcrops to the west. It is grey, inequigranular and hypidiomorphic, with more K feldspar than plagioclase, and >8% quartz. Mafic minerals include amphiboles and biotite and scattered magnetite.
- Andesite Porphyry, characterised by a high plagioclase phenocrysts content. It is typically medium green to grey with a porphyritic texture. Phenocrysts include up to 5 mm subhedral to anhedral plagioclase and scattered up to 3 mm subhedral biotite. Large sporadic, up to 2 mm, plagioclase porphyroblasts are also evident.
• Late Hypabyssal Intrusions, including:
- Hornblende Trachy-Andesite intruding the Sinaloa-Sonora Batholith granodiorite to the west. It is light to medium brown with a porphyritic to trachytic texture, and is characterised by elongated and tabular hornblende with lesser up to 5 mm euhedral to subhedral feldspars and very fine-grained disseminated magnetite.
- Fine-Grained Granodiorite found to the SE. It is light grey, with a composition similar to the more extensive Sinaloa-Sonora Batholith granodiorite. It has a phaneritic texture and usually contains disseminated pyrite, and tourmaline.
- Felsic Dykes that are up to 3 m thick. They are pale pink to light grey, with a porphyritic to aphanitic texture containing sporadic <2 mm subhedral to euhedral biotite and amphiboles, and <1 mm anhedral feldspar. Felsic intrusions were seen in several sites at Santo Tomás, with the most significant intrusion outcrops noted on the El Río Fuerte valley extending NW towards the North Zone. On the basis of intrusive relationships, they are interpreted to of Oligocene-Miocene age, coeval with the Sierra Madre Occidental Volcaniclastics.
- Intermediate to Mafic Dykes, which are typically dark grey, with an aphanitic texture, and in some cases contain fine-grained plagioclase crystals. They have moderate magnetism, containing disseminated and thin veinlet magnetite. They post-date the felsic dykes, suggesting a very young age, possibly linked to Pliocene-Quaternary magmatism represented by scoria cones ~20 km west of Santo Tomás.
• Overburden occurring as unconsolidated and semi-consolidated Quaternary deposits including talus and alluvium valley fill.
Structure
The Santo Tomás and adjacent Brasiles porphyry copper system consists of a 300 to 400 m wide dyke complex with a strike extent of at least 5 km. Mineralisation is strongly structurally controlled, and is associated with the Laramide‐age Santo Tomás fault and fracture zone, a regional‐scale, anastomosing, strike-slip fault zone with a likely long‐lived fault mesh, and several episodes of displacement. This structure controlled the distribution of the quartz monzonite dyke swarm and related hydrothermal alteration, hydrothermal brecciation and sulphide mineralisation. The permeability and fluid flow that produced the alteration and mineralisation in the deposit are the results of multiple faults and an extensive set of fractures related to those faults, that developed in response to hydraulic fracturing. The following structures are evident:
• Reverse faulting is considered the earliest phase of deformation, but while evident in a number of location in the deposit is difficult to recognise due to subsequent reactivation events.
• Sinistral Strike-slip Faulting occurs in two main trends,
- NNE-SSW striking, with dips of 45°WNW to vertical, occurring as transtensional and transpressive structures, matching the orientation of the main Monzonite to Quartz-Monzonite dykes, as well as the system of veins/veinlets within the Santo Tomás porphyry; this orientation also follows the dip and strike of the local stratigraphy, and is consistent with a conformable intrusion of these dykes along the strongly faulted and fractured andesite-limestone contact; and
- ESE-WNW trending, and SSW dipping. This set is subordinate to the previous direction, although it is represented by major fault-shear zones in the Brasiles, North and South Zones that are peripheral to the main mineralisation.
• Dextral Strike-slip Faulting divided into:
- NNW-SSE striking, with dips of 29°WSW to vertical, the dominant set, observed cutting and displacing mineralised structures in the Brasiles area.
- NNE to NE trending, and >50 dipping to the NW and SE.
• Normal Oblique-slip Faults, representing extensional structures which are the most common structures of the area, divided into:
- NNW-SSE trending, the dominant set with 60° dips to both the WSW and ENE to vertical;
- NW-SE trending, dipping at 50° to both the SW and NE, to v ertical;
- NE-SW trending, dipping at >60° to both the NW and SE;
These extensional faults represent the youngest structures in the area, controlling the location of the Intermediate and Felsic dykes, whilst the NNW-SSE normal faulting has been seen to displace an Intermediate dyke.
Within the fault zone controlling the dyke emplacement, quartz monzonite dykes are separated by screens of highly fractured and hornfelsed andesite, and lesser limestone. Related sulphides within this composite fracture zone occur as disseminations, in fractures, and in quartz veinlets.
Alteration
The country rocks at Santo Tomás have undergone both contact metamorphism and hydrothermal alteration. Hydrothermal alteration occurs in zones of hydrothermal brecciation, vein selvages and areas of propylitic, sodic-calcic, potassic, phyllic and argillic alteration styles, as follows:
• Contact Metamorphism has affected most host lithologies, but is most widespread in andesites in contact with quartz monzonite. Andesitic hornfels are aphanitic to very fine‐grained, commonly light‐coloured and mottled with sections of a medium‐ to dark‐grey bleached or slightly waxy, baked appearance. They are also characterised by the absence of identifiable mafic or chlorite grains in the groundmass, compared to the unaltered andesite protolith. These light‐coloured hornfelsed andesites may be slightly albitic, biotitic, potassic or silicified. Hornfelsed andesite commonly has moderate to intense micro‐fracturing with cm to mm spaced fractures.
• Hydrothermal Breccias are developed within Monzonite and Quartz-Monzonite, andesite and andesite tuff units. These breccias are a medium grey and comprise variably sized lithic fragments, usually <8 cm across, set in a cement of quartz, clay, disseminated pyrite and locally tourmaline, and may have a weak to moderate silicification overprint. The shape and thickness of these breccias are highly variable, with the most volumetric recorded in the South Zone with 143.78 and 100 m intersections in drilling. Where developed in andesite, they comprise 2 to 3 and up to 15 cm, well‐rounded clasts, set in a fine‐grained granular matrix that varies from packed clast-supported to matrix‐ supported with 75% matrix. No pervasive hydrothermal alteration is apparent, although some clasts may contain a thin, light grey alteration rim. Hydrothermal breccias in limestone comprise 1 to 10 cm angular, matrix‐supported limestone clasts in a recrystallised limestone matrix that may have been derived from a comminuted limestone rock‐flour matrix, but is texturally similar to the limestone clasts. They have a similar off‐white to light grey colour as massive recrystallised limestones.
• Propylitic alteration, which is the distal or peripheral alteration zone at Santo Tomás. It generally occurs in two stages, the:
- Epidote stage, with an assemblage of epidote, chlorite‐sericite, chlorite‐sericite and chlorite‐epidote‐calcite, with or without albite and quartz;
- Epidote‐free stage, with assemblages that include calcite, chlorite and sericite.
• Sodic-Calcic alteration, interpreted to represent an early‐stage, high‐temperature phase. Sodic‐calcic or albitic assemblages are poorly developed at Santo Tomás, and may be mainly obliterated by later potassic alteration. Where present, it has a uniformly light‐coloured, off‐white groundmass composed of 50% opaque to semi‐translucent albite and 50% transparent to translucent, colourless quartz, occurring as distinct, very fine, separated, sand‐like grains. Minor euhedral remnants of the original plagioclase phenocrysts and quartz phenocrysts attest to a quartz monzonite precursor. Irregular tourmaline patches and minor remnants of chlorite are common. The presence of very localised potassic alteration within the silica‐albite zone may indicate that the albitic alteration of plagioclase phenocrysts and groundmass predate potassic alteration. Silica‐albite alteration is associated with higher than normal sulphide concentrations.
• Potassic alteration occurs along the margins of quartz and quartz‐sulphide veins, but is only visually evident by staining for potassium. K feldspar alteration in andesite locally alters the groundmass from an aphanitic texture to a uniform and very fine‐grained hypidiomorphic granular texture and imparts a slight translucency to the groundmass. The potassic altered andesite is commonly medium grey with no potassium feldspar colouration, whilst individual mineral grains become indistinct and the rock appears to consist of weakly translucent plagioclase and altered plagioclase with minor chloritic remnants after mafic minerals. Staining for potassium reveals abundant speckling, with up to 50 to 60% potassic alteration producing slightly indistinct grain boundaries. Within these zones, the fracture and veining intensity vary from minor to intense with some correlation between higher Cu values and a higher fracture intensity. Locally, the groundmass contains abundant fine‐grained black tourmaline.
Biotite alteration in andesite or hornfels imparts a black to a nearly black colour and produces an aphanitic to very fine‐grained texture. Biotite alteration appears to be less common than other potassic alteration suites.
Potassic altered quartz monzonite is mainly uniform, medium to coarse‐grained with 30 to 35% euhedral to subhedral plagioclase phenocrysts, 10% quartz phenocrysts and 5% chlorite after primary hornblende. Locally it is in part porphyritic with a relatively fine‐grained groundmass and may be transitional to feldspar and quartz‐feldspar porphyry. The altered plagioclase phenocrysts are chalky, off‐white, and partly kaolinised, to pale pinkish when strongly potassic altered.
• Phyllic alteration has a restricted occurrence at Santo Tomás. Where subjected to phyllic alteration, the quartz monzonite is uniformly medium grey, slightly translucent, with a weak colour contrast between plagioclase phenocrysts and groundmass. It has the appearance of being silicified, but scratches with a knife, and is slightly waxy as opposed to lustrous. Chlorite is developed after hornblende, indicated by grains that are less euhedral and less distinct. Much of the phyllic alteration described in the core at Santo Tomás appears to be confined to the zone of early‐stage faulting and fracturing and the quartz monzonite intrusion.
• Argillic alteration has only been described in core as occurring in fault zones in intrusive and andesitic rocks, but it is generally not described in the adjacent lithologies, suggesting it refers to fault gouge rather than a zone of pervasive hydrothermal alteration.
Mineralisation
The Santo Tomás – Brasiles porphyry copper system comprises a 300 to 400 m wide dyke complex with a gently curved strike length of at least 5 km. Over that strike length, it is divided into three segments, partially separated by lower grade intervals, namely: i). the South Zone, the southern section of the generally west-dipping tabular deposit, largely on and under the western slope of the north-south Santo Tomás Ridge; ii). the North Zone, the main mineralisation, which lies on the eastern flank of the Santo Tomás Ridge, and extends westward towards the western bounding valley under post mineral rocks and limestones; the eastern expression of the mineralisation in this zone occurs as oxide copper mineralisation at surface, hosted by quartz monzonites and andesites; iii). the Brasiles Zone on the opposite, northern, side of the Río Fuerte which marks the border with Chi. The curved strike varies from NE-SW in the North Zone, to SE-NW in the South Zone. Mineralisation has a strong structural controll and is associated with the NNE-SSW striking, 45°WNW dipping to vertical Laramide‐age Santo Santo Tomás fault and fracture zone (an 'Early-Stage Structural zone') which controlled the distribution of the 57.2 ±1.2 Ma monzonite and quartz monzonite dykes swarm, and related hydrothermal alteration, hydrothermal breccias and sulphide mineralisation. Mineralisation is hosted in both the intrusions and strongly faulted and fractured Cretaceous meta-andesite and limestone units, the contact of which is followed by the dyke swarm.
The main mineralisation forms a broadly tabular zone that varies between ~100 and 400 m, and locally up to 600 m in true thickness, and dips moderately at ~50°WNW in the North Zone, whilst similar moderate dips are also apparent in the southerly portion of the South Zone where mineralisation dips sub-parallel, or slightly steeper than, the west‐facing slope of the Santo Tomás ridge. Interpretation of the moderate westerly dip of the mineralised zone is complicated by stepwise down‐dropping of the mineralised zone along the west side of both the North and South Zones, due to the influence of late faulting of the Western Fault zone.
The Santo Tomás mineralogy comprises chalcopyrite, pyrite and lesser molybdenite with minor bornite, covellite and chalcocite distributed within the quartz monzonite dykes and altered andesite country rocks. The tabular mineralised body is primarily defined by veins/veinlets, finely disseminated sulphides and fracture‐fillings with subordinate sulphides hosted in stockwork quartz veinlets, together with K feldspar, biotite-potassic, quartz-sericite, calcite, chlorite, and locally, tourmaline alteration, as described in the alteration section above. Minor copper oxides occur near the surface. Minor skarn and replacement‐style mineralisation in found in the hanging wall limestone.
Nine vein types have been discerned at Santo Tomás, with the main copper bearing mineralisation related to the Early Dark Micaceous (EDM), A and B type veins. These are:
- Type M early magnetite veins;
- Type LG veins containing quartz, biotite, chlorite ±magnetite;
- Early Dark Micaceous (EDM) veins, hosted in the Quartz Monzonite and characterised by dark mica, including muscovite with secondary biotite, accompanied by anhydrite, quartz and chalcopyrite, typically with a coarse subhedral biotite selvage.
- Type A veins, which usually have diffuse margins and contain quartz, chalcopyrite ±bornite ±magnetite, and are characterised by anhydrite-bornite with a chalcopyrite median line, and disseminated biotite-magnetite.
- Type B veins, which cut the Type A veining and are hosted within both the quartz monzonite and andesite. They are the most common and important veins. They often have a central suture containing fine molybdenite and grains of chalcopyrite and vary from mms to cms in thickness. Proximal to the main mineralised centre, they comprise quartz-chalcopyrite with fine molybdenite on the vein margins and central suture, and have a K feldspar-illite selvage. Where distal, they instead are composed of quartz with molybdenite-chalcopyrite-pyrite, and an illite/green sericite selvage.
Zones with high densities of type A and B veins are characterised by chalcopyrite-molybdenite and minor bornite, and are typically associated with the potassic and phyllic alteration zones. Preliminary fluid inclusion analysis indicate the A-B vein types are associated with fluids containing up to 53.6 wt.% NaCl and temperatures of up to 442°C (Norine et al., 2023).
- Type D veins, with quartz, sericite and pyrite;
- Type E veins contain quartz, sericite, pyrite and carbonate ±sphalerite ±galena, and are late stage, cutting both A and B veins. These are characterised by a quartz-calcite pyrite with associated chalcopyrite-sphalerite in proximal and sphalerite-galena in distal zones.
- Type DX veins, containing a pyrite, cobalt/pyrite, chlorite assemblage; and
- Type QTP veins, with quartz-tourmaline-K feldspar ±pyrite mineralogy.
Although largely controlled by structures related to pre-mineral sinistral NNE-SSW striking faults, most mineralised veins apparently are within fractures which at the time of deposition, indicate a dextral component of movement influencing the development of dilation and open space to facilitate deposition. Locally, layered/laminated types A and B veins contain accumulation of fine sulphides layers, indicating a crack-seal process and a dynamic environment with multiple re-opening of the same fracture system, filled by the same hydrothermal source.
Chalcopyrite is the principal copper mineral, occurring with pyrite, both as fine‐grained disseminations throughout altered rock types, and within the central or marginal parts of microfractures that have been filled with quartz and potassium feldspar, and locally with black tourmaline crystals, both within the intrusive and adjacent andesite. The microfractures host 1 to 3 mm thick quartz and potassium feldspar veins. The hairline fractures produce surfaces with scattered very fine sulphide grains that have the appearance of randomly disseminated grains. Some 1 to 3 mm quartz‐sulphide veins contain chlorite or magnetite.
Mineral Resources
Previous published reserves include:
>80 Mt @ 0.55% Cu (Res., 1975, Sillitoe, 1976).
Mineral Resources at an in-situ cut-off grade of 0.15% Cu, as at 21 April, 2023, after SRK Consulting (US), Inc. as quoted by Norine et al., 2023, were:
Indicated Resource
North Zone - 487.3 Mt @ 0.36% Cu Equiv., 0.32% Cu, 0.009% Mo, 0.03 g/t Au, 2.1 g/t Ag;
Inferred Resource
North Zone - 197.1 Mt @ 0.36% Cu Equiv., 0.32% Cu, 0.005% Mo, 0.03 g/t Au, 2.1 g/t Ag;
South Zone - 402.8 Mt @ 0.35% Cu Equiv., 0.31% Cu, 0.008% Mo, 0.02 g/t Au, 1.9 g/t Ag;
TOTAL - 599.9 Mt @ 0.36% Cu Equiv., 0.32% Cu, 0.007% Mo, 0.03 g/t Au, 2.0 g/t Ag;
Indicated + Inferred Mineral Resource
TOTAL at 0.15% Cu cut-off - 1087.1 Mt @ 0.36% Cu Equiv., 0.32% Cu, 0.008% Mo, 0.03 g/t Au, 2.0 g/t Ag.
TOTAL at 0.3% Cu cut-off - 516.7 Mt @ 0.47% Cu Equiv., 0.42% Cu, 0.009% Mo, 0.03 g/t Au, 2.4 g/t Ag.
TOTAL at 0.4% Cu cut-off - 240.4 Mt @ 0.56% Cu Equiv., 0.50% Cu, 0.008% Mo, 0.04 g/t Au, 2.9 g/t Ag.
The information in this summary has been drawn from "Norine, J.A., Mehrfert, P., Burkett, S. and Uken, R., 2023 - Santo Tomás Project, Mineral Resource Estimate; an NI 43-101 Technical Report prepared by Ausenco Engineering USA South Inc.; for Oroco Resource Corp., 284p." Uken, R., was responsible for the Geological sections of the report.
The most recent source geological information used to prepare this decription was dated: 2023.
Record last updated: 27/7/2023
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.
Santo Tomas
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Valencia-Moreno, M., Ochoa-Landin, L., Noguez-Alcantara, B., Ruiz, J., and Perez-Segura, E., 2007 - Geological and metallogenetic characteristics of the porphyry copper deposits of Mexico and their situation in the world context: in Alaniz-Alvarez, S.A. and Nieto-Samaniego, A.F., (Eds.), 2007 Geology of Mexico: Cele brating the Centenary of the Geological Society of Mexico: Geological Society of America, Special Paper 422, pp. 433-458.
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