Hermosa - Taylor, Clark
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The Taylor and Clark (previously Central) zinc, lead and silver deposits of the Hermosa Project are part of the Harshaw and Patagonia Mining Districts in the Patagonia Mountains of Santa Cruz County, Arizona, USA. They are ~90 km SE of Tuscon, 8.5 km SSE of the town of Patagonia and 24 km NE of Nogales on the Mexican border. Clark is the southeastern, shallower and up-dip oxide zone of the stratabound Taylor Sulphide manto, which is, in turn, offset by a flat northeast vergent thrust from the underlying Taylor Deeps sulphide ore to the NW.
(#Location: 31° 27' 46"N, 110° 43' 10"W).
The Hermosa property lies ~1.6 km SE of the historic Trench zinc-silver-copper mine that was operated by ASARCO between 1939 and 1949 exploiting a composite steep, transgressive fissure vein sulphide deposit. The production from this mine, including the earlier period from 1850 to 1890, totalled ~0.215 Mt @ 445 g/t Ag, 8.5% Pb, 6.3% Zn. Earlier historic mining in the Harshaw district dates from the mid-18th century Spanish Colonial times. Oxide lead-silver veins had been mined until the late 19th century at the Trench mine area and from the Mowry Property ~3 km to the south of Hermosa, and a number of mines in between, until the late 19th century. These included the Alta Claim, pegged in 1877 which produced ~3200 t of oxidised lead-silver material grading ~350 g/t Ag, 35% Pb, 1% Cu from a NE-dipping vein. The Hardshell Incline Mine, 900 m to the NW of Hermosa, was discovered in 1879 and produced ~32 000 t @ 250 g/t Ag and 6 to 8% Pb between 1896 and 1964. The Hermosa Mine, discovered about the same time as the veins of the Hardshell Incline Mine, produced high-grade silver halide mineralisation from a 30° north-dipping stratabound vein, averaging ~685 g/t Ag. Approximately 63 490 tonnes of ore was processed over an 18 to 24 months period to produce 43.5 t of silver, whilst secondary scavenging from 1902 to 1943 recovered another 18.5 t. This production was by small-scale miners working a number of individual small mines in the Hermosa area extracting small tonnages of milling and direct-shipping oxidised mineralisation.
The Flux Canyon deposit, 3 km NW of Trench was drilled and mined by ASARCO as flux and additional feed to their Trench smelter, initially in oxides but by 1944, at depths of ~130 m, Pb-Zn-Ag sulphide ore was becoming the primary economic target (Kartchner, 1944). Approximately 1.5 million tonnes of ore was mined from the deposit between 1950 and 1962 at a combined grade of 7% Pb+Zn and 140 g/t Ag (Arizona Geological Survey, quoted by Turner, 2017).
ASARCO undertook intermittent drilling campaigns on the Hermosa property between 1940 and 1991, but failed to outline significant extensions to the Hardshell Incline lead-silver minerals. Lead-silver oxide mineralisation was shipped from the lower levels of the Hardshell Incline Mine from 1943 to 1948 and between 1963 and 1964, amounting to 2250 and 2630 t respectively, both grading 275 g/t Ag, 6% Pb. Second pass diamond drilling between 1946 and 1953 located thick Ag-Pb-Zn bearing, manganese oxides of the Main Hermosa Manto to the southeast of the Hardshell Incline. Over the years ASARCO made a number of resource and reserve estimates (all pre NI 43-101) that included 5.9 Mt @ 170 g/t Ag, 1% to 2% Pb+Zn, 15% MnO2 in 1968; 18 Mt @ 115 g/t Ag, 8% Mn at a 2:1 open pit stripping ratio in 1975; 5.9752 Mt @ 270 g/t Ag, at a 170 g/t Ag cut-off in 1979; and 8.705 Mt @ 236 g/t Ag, at a 51 g/t Ag cut-off in 1984. Pan American Silver had an option on most of ASARCO’s Hardshell Property from 1994 to 2002, but did not undertake any significant exploration work, other than economic evaluations.
The Clark and Taylor deposits, as described below, were outlined by the Canadian company Arizona Mining Inc. through its wholly owned Arizona registered subsidiary Arizona Minerals Inc. In October 2005, Arizona Minerals Inc entered into an agreement with ASARCO to purchase the bulk of the Hermosa property, and in January 2016 closed the acquisition. From 2006, Arizona Minerals carried out a re-assay program of all remaining ASARCO assay pulps, which had been included in the option, to verify results, build a high quality database and undertake a Mineral Resource estimate and preliminary economic evaluation. This was followed by detailed geological and structural mapping and a drilling program. Drilling was initially focused on the oxide stratabound manto mineralisation of the Central Deposit between 2007 to 2012 leading on to the underlying and down plunge Taylor Sulphide Deposit from 2010, but mainly from 2014 to determine the extent of the carbonate replacement stratabound manto mineralisation. The Taylor Deeps were first reported in reports of 2017. In August 2018 South32 completed a 100% acquisition of the project.
The tectonic setting of southwestern United States and the northern half of Mexico may be broadly divided into three generally NW-SE aligned belts:
i). a western to southwestern region of largely Mesozoic arc related rocks overlying Palaeozoic sedimentary rocks, predominantly in Mexico. This belt is now found on the Baja California Peninsula and the coastal strip of Sonora and Sinaloa which were contiguous prior to the Miocene opening of the Gulf of California.
ii). a mainly Palaeoproterozoic basement in eastern California, Arizona and New Mexico in the U.S., and northern Sonora and much of Chihuahua in northern Mexico. East of California and Nevada, where other tectonic factors intervene, this block is further divided into two NW-SE trending terranes:
• a suite of meta-volcanic rocks, in the southwest of this terrane pair, mostly in SW Sonora, separated from its neighbour to the NE by a major, NW-SE trending sinistral structure, the Mojave-Sonora Megashear.
• a Late-Palaeoproterozoic (1.72 to 1.68 Ga) terrane dominated by clastic meta-sedimentary rocks and intruded plutons, which occupies much of Arizona, New Mexico and northern Sonora.
During the late Neoproterozoic and early Palaeozoic these two Precambrian terranes were split into a number of rift basins with passive margins. These were filled with rift sediments which passed up into a succession of Phanerozoic rocks comprising Palaeozoic platform cover, predominantly carbonate and lesser clastic sedimentary rocks, a variable thickness of Mesozoic clastic and volcanic rocks, and by Mesozoic shelf carbonate rocks to the southeast in Mexico. This variably eroded sequence is 2 to 4 km thick.
iii). an eastern to northeastern series of thick Palaeozoic and Mesozoic shelf sequences overlying a largely concealed Precambrian metamorphic basement. In Mexico these are overlain by a Mesozoic carbonate platform built on similar Palaeozoic and Early to Mid-Mesozoic clastic sequences. This carbonate platform is part of the similar domain described in the Precambrian blocks above, and incorporates the Oaxaquia Terrane in central and eastern Chihuahua and the Colorado Plateau and eastern Great Basin the US (Barton et al., 1995).
The southwestern North America continent had a passive margin until the onset of the Antler Orogen during the late Devonian to early Carboniferous, followed by the Sonoma Orogeny in the Late Permian to Early Triassic. The focus of both was well to the NW in Nevada and involved the SE vergent collision and overthrusting of allochthonous blocks from the NW, with SW Arizona lying within the shallow carbonate shelf of the Interior Platform and foreland basin, well inland of the main continental margin. Southwestern North America became an active margin during the Late Triassic with NE directed subduction just to the SW of the Baja California-Sinaloa coast. Magmatism associated with this active margin is evident as a major Mesozoic arc above the subduction zone, as described in i). above. However, magmatism also occurred in a broad belt as much as 100 to 200 km wide that is 350 to 400 km inland of the active continental margin. This belt occurs as far inland to the NE as eastern Nevada and western Utah, southern Arizona and southwestern New Mexico in the US and eastern Sonora and Sinaloa in Mexico, gradually younging and converging with the active margin to the south. In doing so, it transgresses across all three of the basement blocks described above. There are three main periods of activity with which mineralisation is associated within this broad belt. The oldest is of Jurassic age in southeastern Arizona and western Nevada, then a second from 110 to 105 Ma in Eastern Nevada. The third was by far the most extensive, related to the Late-Cretaceous to Late-Eocene (80 to 50 Ma) Laramide Arc which generally younged to the SW and south. All three magmatic episodes are represented in the Hermosa district. Finally, all three basement blocks described above, but largely in Mexico, are overlain by the 1200 x 300 km sheet of post-Laramide Eocene to Miocene silicic volcanic rocks, predominantly ignimbrites, of the Sierra Madre Occidental corresponding to a period of extension, as described below in the Structure section.
The Hermosa property lies to the NE of the Mojave-Sonora Megashear, within the northeastern of the two basement terranes that constitute the Proterozoic block that is the second of the three main tectonic belts. It is underlain by late Palaeoproterozoic rocks, dominated by the Pinal Schists, a greenschist facies sequence of metamorphosed argillaceous quartzwacke (Anderson, 1989) that were unconformably deposited on older (>2 Ga) Palaeoproterozoic basement, and intruded by Mesoproterozoic ~1450 Ma granitic stocks and batholiths (Silver et al., 1977). It also lies within the inland Mesozoic the Tertiary magmatic arc that hosts almost all of the significant porphyry copper deposits of SW North America, but also represents the northern extension of the Mexican Silver Belt. For background on the latter, see the Regional Setting section of the Fresnillo deposit record.
The stratigraphic sequence in the Patagonia Mountains District comprises, from the base (after Graybeal et al., 2015, Methven et al., 2018):
Basement - granodiorite with subordinate amounts of pelitic schist, diorite and gabbro.
Bolsa Quartzite, ~72 to 81 m thick - the unconformably overlying lowermost unit of the Cambrian sequence, commencing with a basal quartzite pebble and boulder conglomerate that is a few cm to 5 m thick, overlain by brown to light-grey, gritty to pebbly quartzite and lesser sandstone. Shaly siltstone partings generally <2 cm thick, occur in the upper two-thirds of formation;
Abrigo Formation, ~260 m thick - comprising a grey, brown or purple, coarse-grained, thick- to thin-bedded limestone, dolomitic limestone, silty limestone and lesser limestone conglomerate.
El Paso Limestone which is the only significant Ordovician unit in southeastern Arizona, that was largely emergent during the Ordovician and Silurian. This unit may be as much as 133 m thick and comprises limestone and/or dolostone which generally contain scattered lenses and nodules of chert;
Martin Formation, which is the prevalent Devonian unit in the southern part of Arizona, disconformably overlying the Abrigo Limestone. It is of Late Devonian age, 90 to 120 m thick, and is composed of a lower member of conglomeratic sandstone and dolomitic limestone, a middle member of sandstone and cliff-forming limestone, and an upper member of sandstone, sandy limestone and shale.
Percha Formation, also of Late Devonian age, composed of a 35 to 74 m thick series of shales overlain by interbedded dolostones and limestones.
Escabrosa Limestone, of Lower Carboniferous (Mississippian) age, which is ~142 m thick and comprises a light grey, coarse- to fine-grained, thick- to thin-bedded cherty limestone and lesser dolomitic limestone that disconformably overlies the Martin Limestone.
UPPER CARBONIFEROUS to PERMIAN
Naco Group, composed of the:
• Horquilla Limestone, which is of Upper and Middle Pennsylvanian age and unconformably overlies the Escabrosa Limestone. It is ~82 m thick and composed of light-grey, grey, or pinkish-grey, fine- to coarse-grained, medium-bedded limestone and lesser dolomitic limestone, and brown to maroon thin-bedded limestone.
• Earp Formation of Upper Pennsylvanian to Lower Permian age and ~230 m thick. It is composed of grey, light-grey, or pink thin-bedded to massive, sandy to silty limestone and dolomitic limestone, with lesser dolostone, chert and limestone, conglomerate and sandstone.
• Colina Limestone, of Lower Permian age, which is 70 to 105 m thick, comprising grey to dark-grey, fine-grained, and medium- to thin-bedded limestone with thin beds of dolostone.
• Epitaph Dolostone of Lower Permian age and ~260 m thick, composed of grey fine-grained, thick-bedded limestone, silty limestone, grey dolomitic limestone, lesser sandstone and conglomerate, with sparse pods of chert and quartz. On the Hermosa property it comprises three lithofacies, namely:
- Limestone - grey, bleached, massive to irregularly thin-bedded, very fine-grained limestone with rare, irregular, dark grey to black, 1 x 5 cm to 10 x 25 cm chert pods with 1 to 10 mm thick talc rims. It also contains common 1 to 25 mm spots, pods and ovoids of white calcite after gypsum;
- Silty Limestone, which is grey, thin-bedded, very fine-grained and silty, with well preserved, regular, 0.1 to 1 mm thick beds with common carbonaceous microfaults and partings containing very-fine-grained, pyrite. It is calcareous and has frequent short intervals without thin-bedding.
- Carbonaceous Limestone which is dark grey to black, and massive to thin-bedded.
• Scherrer Formation of Lower Permian age and ~45 m thick, comprising brownish-grey to grey, massive, sandy limestone and white to light-brownish-grey, fine-grained sandstone. On the Hermosa property it is composed of three lithologic members:
- Lower Member of massive to thin-bedded grey non-calcareous quartzite, with 60% fine-grained, well-rounded, well-sorted quartz grains;
- Middle Member of massive to irregularly thin-bedded limestone which includes variations of silty and cherty limestone;
- Upper Member - a light grey, massive, calcareous sandstone with sparse, relict thin bedding and 30 to 60% fine-grained, well-rounded, well-sorted quartz sand in a calcareous matrix;
• Concha Limestone of Lower Permian age and ~155 m thick, composed of grey to light-grey, fine-grained, medium- to thick-bedded limestone with lenses and nodules of chert. On the Hermosa property it is grey, massive, fine-grained, recrystallised limestone-marble with common, irregular dark grey to black, 1 x 5 cm to 10 x 25 cm chert pods and local 1 to 5 mm thick, irregular, discontinuous calcite veinlets. The Concha Formation limestone-marble is distinguished from the underlying Scherrer Formation by the presence of prominent chert nodules and the complete absence of sandy detritus.
TRIASSIC to JURASSIC
Older Volcanic Sequence, also known as the Mount Wrightson Formation, which is 600 m or more in thickness, comprising an Upper Triassic to Jurassic suite that disconformably to unconformably overlies the Palaeozoic Epitaph, Scherrer and Concha formations. It is a predominantly rhyolitic volcanic package, outcropping in the southeastern part of the Hermosa property, containing lithologies that are recognised in clasts in the overlying Hardshell Volcanic Sequence. It includes the following units in approximate stratigraphic order (after Methven et al., 2018):
• Tuffaceous Sandstone, which unconformably overlies the Palaeozoic sequence and comprises a tan to reddish-brown, granular, fine-grained, massive to thin bedded, reworked, partially silicified tuffaceous-sandstone, composed of fine-grained quartz and feldspar with sparse lithic fragments.
• Basal Breccia, which is either a basal unit or a structural artifact. It contains abundant, angular to sub rounded, 50 mm to greater than core diameter, clasts of lithic fragments of older volcanics, tuffaceous sandstone, limestone, sparse limestone clasts replaced by pyrite, and frequent to abundant quartz and calcite veins.
• Lithic Tuff, a greenish-grey, fragmental volcanic with rare, 1 to 3 mm, subhedral plagioclase phenocrysts and common 1 to 25 mm, angular, lithic clasts set in a fine-grained, partially silicified, tuffaceous groundmass.
• Latite Porphyry, which is a distinctly porphyritic intrusive and/or flow unit with prominent and abundant, white, subhedral to euhedral, 1 to 5 mm prismatic plagioclase phenocrysts, and less commonly 1 to 5 mm, euhedral, white, approximately equant K feldspar phenocrysts. Rare, relict, 0.1 to 1 mm, rotten, biotite books are set in a fine to medium-grained, red-brown groundmass.
• Rhyolite Welded Tuff, a light reddish-grey to purple, densely welded crystal tuff with strong to subtle laminar eutaxitic texture and abundant, 0.1 to 3 mm, subhedral to euhedral, plagioclase phenocrysts in shard-bearing, eutaxitic, very-fine-grained groundmass. It is laminated to thin-bedded, which is locally contorted due to flowage. It is the most common clast lithology in sections of the Hardshell Volcanic Sequence.
• Rhyolite Spherulite Zone, composed of abundant, crowded, 1 to 100 mm, semi-spherical, zoned, partially devitrified spherulites in a very fine-grained partially welded groundmass.
To the NW of the Hermosa property, it is commonly pyritic and heavily altered to quartz, sericite, kaolinite, alunite, tourmaline and pyrophyllite to the west, SW and south of the Sunnyside breccia porphyry system. This altered and pyritic zone is ~2 km to the west of the Hermosa deposits.
Hardshell Volcanic Sequence, which is of Jurassic age, unconformably overlying both the Older Volcanic Sequence and the Palaeozoic succession. Five distinct rhyolitic volcanic units have been identified, in addition to a basal Tuffaceous Sandstone, as follows (after Methven et al., 2018):
• Tuffaceous Sandstone, a tan to reddish-brown, granular, fine-grained, massive to thin bedded, reworked, partially silicified tuffaceous-sandstone, composed of fine-grained quartz and feldspar with sparse lithic fragments.
• Rhyolite Tuff, a light grey, massive, rhyolite tuff with rare, fine-grained plagioclase phenocrysts and <10 mm lithic clasts set in a very-fine-grained, tuffaceous groundmass. Local irregular, faint, relict bedding and weak, hematite-limonite liesegang banding are also evident. It lies directly on Palaeozoic sedimentary rocks in the western part of the property, and on the spherulite unit of the Older Volcanic Sequence to the east.
• Rhyolite Polymict Breccia, the unit that is the primary host to the deposits exploited by the old Hardshell Incline mine. It is a rhyolitic volcaniclastic and fragmental that has abundant 1 to 25 mm, angular, rhyolite lithic clasts set in a welded, eutaxitic matrix, and is distinguished from the rhyolite tuff unit by the presence of variably abundant Palaeozoic sedimentary clasts. It commonly contains 1 to 10 mm blebs, veins, veinlets, fracture coatings and larger completely replaced pods of Mn-oxide. It also includes limestone clasts up to 3 m in diameter that are replaced by Zn-Pb-Ag sulphides, at depth, in the northwest part of the property.
• Rhyolite Lithic Tuff, a grey to grey-green, locally crystal-rich tuff which commonly contains 5 to 25 mm rhyolitic lithic fragments. It also contains abundant 1 to 25 mm, partially-collapsed and flatted pumice fragments in very fine-grained, partially welded groundmass that gives the rock a distinctive, eutaxitic texture.
• Rhyolite Breccia, that forms a prominent outcrop in the Hardshell Ridge zone. It is a clast-supported to 'almost clast-supported' fragmental unit with abundant angular, unsorted, rhyolite clasts that range from 1 mm to 5 m across set in very-fine-grained rhyolitic groundmass.
• Rhyolite Crystal Tuff, that appears to be the uppermost unit in the Hardshell Volcanic Sequence and is conformable with the underlying rhyolite breccia unit. It is a white to grey to buff to locally pale pink, fine- to medium-grained crystal-rich tuff, with rare, thin, relict bedding planes. It contains abundant 1 to 3 mm plagioclase crystals and rare 0.5 mm, broken quartz eyes, as well as rare patches and zones of 5 to 15 mm, angular to subrounded lithic clasts.
Jurassic equigranular granite, as well as hornblende monzonite dated at ~173 Ma (U-Pb zircon); 160 ±20 Ma (Pb zircon) granite, porphyritic granite and 150 ±20 Ma equigranular alkali syenite are found 4 to 5 km to the west of Hermosa. These together occur in the western half of the composite batholithic complex that forms the core of the Patagonia Mountains (see below) and irregularly intrudes Proterozoic basement on its western margin.
Bisbee Formation, which is up to 900 m thick and of Lower Cretaceous age. It is found to the west of the Hermosa deposits where it is in faulted contact with the wedge of mineralised Palaeozoic carbonate sequence to its east. It is mainly a dark- to light-grey siltstone and mudstone, with some sandstone, limestone and conglomerate. It has been weakly to strongly hornfelsed and is commonly pyritic, and disconformably overlies Jurassic silicic volcanic rocks.
Meadow Valley Trachyandesite and Andesite, is a complex flow unit that more or less conformably overlies the Hardshell Volcanic Sequence on the western and northern margins of the Hermosa Property, and is underlain by local dykes of similar composition. It is dark grey to brown, fine to medium-grained with 1 to 3 mm euhedral to subhedral plagioclase phenocrysts and sparse 2 to 5 mm square K feldspar phenocrysts, all set in a fine-grained plagioclase-pyroxene-amphibole matrix. It contains variable interstitial magnetite and is generally fresh to weakly propylitic altered, particularly on fractures.
Upper Cretaceous pyroxene monzonite and porphyritic biotite granodiorite dated at ~74 Ma (U-Pb zircon) occur in the batholithic complex that forms the core of the Patagonia Mountains (see below) a few km to the west and may be intrusive equivalents of the Meadow Valley volcanic rocks.
CRETACEOUS and TERTIARY
Quartz-Feldspar Porphyry Intrusion, primarily occurring in the Taylor Sulphide section of the deposit as narrow dykes occupying high angle structures cutting through the Palaeozoic sequence, and as narrow sills following the contact between the Concha and Scherrer formations.
Diorite Intrusion, which is most commonly found below the 'Lower Thrust' fault between the Taylor Sulphide and Taylor Deeps domain at depths of >1036 m below surface, possibly acting as feeders to the Meadow Valley Trachyandesite.
Neither intrusion had been age dated (in 2018).
Granodiorite of the Patagonia Mountains, that forms much of the composite batholithic core to the range and passes within ~2 km to the west of the Hermosa deposits, although a number of small intrusive bodies are found in the intervening interval. While much the batholith is composed of this Laramide granodiorite, it also includes significant volumes of Jurassic and Cretaceous granitoid intrusions as described above. This composite Jurassic to Paleogene batholithic intrusion is ~6 km wide at the Mexican border, and by 20 km to the north, has tapered to a series of NNW elongated irregular plugs and dykes near the Red Mountain porphyry copper deposit that is 5 km NNW of the Hermosa property. It also hosts the Sunnyside porphyry/breccia Cu resource <2 km NW of Hermosa.
The 'granodiorite' of the Patagonia Mountains, which is predominantly of Paleocene age, includes a
64 to 60 Ma (U-Pb zircon and 40Ar/39Ar biotite) biotite-quartz monzonite;
a 64 to 60 Ma (U-Pb zircon and 40Ar/39Ar biotite) biotite granodiorite;
a 64 to 60 Ma (U-Pb zircon and 40Ar/39Ar biotite) quartz monzonite;
~60.1 ±0.9 Ma (U-Pb zircon) quartz monzonite porphyry;
a 58 ±5 to 63.9 ±2 Ma (K-Ar biotite and hornblende) granodiorite;
a latite porphyry; syenodiorite or mangerite; a biotite augite quartz diorite;
a 59.94 ±0.85 to 60.0 ±1.4 Ma (U-Pb zircon) quartz-feldspar porphyry, and intrusive and intrusion breccias.
The quartz-feldspar porphyry and diorite dykes within the immediate deposit Hermosa deposit area, as described above, belong to the same intrusive complex. This batholithic intrusive complex is bounded by NNW to NW-striking faults and its emplacement is thought to have been structurally controlled, occupying a significant structural zone between the Proterozoic basement to the west and Palaeozoic-Cenozoic rocks to the east. Laramide felsic volcanic and intrusive stocks are prevalent at Red Mountain and west of the historic Trench mining camp in the Sunnyside deposit area. Intrusive rocks and alteration at Sunnyside are thought to be coeval with alteration at the Hermosa property (Methven et al., 2018).
Paleocene Intrusive breccia, found immediately to the NW of the Hermosa property, occurring as greyish, compact rock of the Sunnyside porphyry Cu-Mo system with matrix-supported, angular clasts of Mesozoic volcanic and sedimentary rocks cemented by fine-grained quartz and other minerals (Graybeal et al., 2015).
Paleocene volcaniclastic rocks, also referred to as the lapilli tuff, covering a roughly circular, ~1.6 km diameter area at surface, which appears to post-date the quartz-monzonite porphyry, but is cut by several quartz feldspar porphyry intrusions. It is located immediately NE of, and overlapping the Sunnyside porphyry deposit and immediately NW of Taylor Deeps. It is a greyish to white, well consolidated and poorly sorted lapilli tuff and tuff breccia. Lithic clasts in the lapilli tuff are generally angular, <2.5 cm in diameter, and largely comprise Mesozoic volcanic and sedimentary rocks. These are accompanied by 35%, mostly broken, clear quartz crystals with strongly resorbed boundaries on preserved crystal faces. Minor fragmented feldspar phenocrysts are also present. The lapilli are matrix supported and form about 30% by volume of the lapilli tuff, and with the crystals and lithic clasts, are set in a fine-grained, illite-alunite-kaolinite-altered matrix, interpreted to be after ash. It also contains numerous silicified zones, both transgressive and confirmable. The bedded sequences have concentric strike and inward dips. On the outer margins dips are shallow, but toward the centre of the exposed mass dip at up to 70°. Drilling shows it persist to depths of as much as 1 km below the surface with steeply dipping walls and at depth may become a xenolithic-rich intrusIon. It is interpreted to occupy a >0.7 km diameter vent formed as a result of explosive fragmentation of rising volatile-rich magma during a phreatomagmatic eruption. Intrusive breccia masses along the southwest side of these volcaniclastic rocks are texturally similar to the volcaniclastic rocks and contain locally abundant quartz monzonlte porphyry clasts up to 30 cm across (Graybeal 1996; Graybeal et al., 2015).
Late Oligocene to Miocene, conglomerates, sandstones, ash flow tuffs and lake bed sedimentary rocks of the San Rafael Basin to the east of the Patagonia Mountains and the northeastward-trending Sawmill Creek Basin lap onto the Hermosa Property.
The stratabound Taylor Deeps and Taylor Sulphide sections of the deposit, as well as the manto which was oxidised to form the Clark deposit, are all hosted within the carbonate sedimentary rocks of the Lower Permian Concha, Scherrer and Epitaph formations, immediately below the disconformity at the base of the Jurassic Hardshell Volcanic and/or Upper Triassic to Jurassic Older Volcanic Sequence. The uppermost of these Permian units, the Concha Formation is the principal host, although in the structurally more complex Taylor Sulphide section of the deposit, mineralisation also extends into the underlying Scherrer and Epitaph formations. Volcanic hosted manto and steep transgressive vein mineralisation are found above the carbonate hosted mantos within the Mesozoic volcanic rocks of the Triassic and Jurassic Older and Hardshell volcanic sequences and the Cretaceous Meadow Valley Andesite. These latter mineralisation styles are interpreted to be coeval with and related to the same event that produced the Permian carbonate hosted mantos (Methven et al., 2018).
The current regional structural framework of Arizona was largely established during the late Mesozoic and Tertiary, although there is evidence that Proterozoic structures were reactivated during this time (Krantz, 1989). Three post-Lower Cretaceous stages of magmatism and deformation are recognised in southern Arizona, namely the Late Cretaceous to Early Tertiary Laramide, Mid-Tertiary Orogeny and Late Tertiary tectonic Basin and Range phases.
The Laramide Magmatic Arc was developed over the period from ~80 to 50 Ma and was responsible for most of the porphyry type copper mineralisation in the southwestern U.S. and northern Mexico. Laramide igneous activity was accompanied by an ENE-WSW directed compressional tectonic regime, resulting in thrusting and reverse faulting (Rehrig and Heidrick, 1972; Heidrick and Titley, 1982). The cessation of Laramide plutonism, volcanism and crustal shortening, by around 50 Ma in the Mid-Eocene, was followed by a 15 m.y. period of magmatic quiescence, erosion and continental sedimentation, the Eocene Epeirogeny.
From approximately 35 Ma, a period of post orogenic extension ensued over the area of Precambrian basement, persisting from the Oligocene to the Mid-Miocene. This phase was characterised by the development of listric, detachment and strike-slip faults, associated listric tilting, and the buoyant rise and exposure of metamorphic core complexes. In some cases it is envisaged that extensional detachment faulting exploited earlier compressional thrusts via their reversal. Tilting of pre-Oligocene strata by the listric faulting has been measured at as much as 60°, and extension on individual detachments have been measured at from 10 to 70 km directed along axes generally oriented in a direction of 55 to 70° in Arizona and Sonora. Post Laramide magmatism was initiated at ~35 Ma, in the Oligocene, and persisted for 15 to 20 m.y. to the Early Miocene, accompanying the extensional regime described above. This magmatism and structural activity comprised the 'Mid-Tertiary Orogeny'. Magmatic activity is represented by voluminous ignimbrite-dominated volcanism in the Sierra Madre Occidental.
During the Mid- to Late-Miocene, between 18 and 10 Ma, the nature of tectonism in the area previously occupied by the Laramide Arc changed again, from listric extension, to a normal block faulted regime which persists to the present in some areas. The change also coincided with the cessation of the intense Mid-Tertiary terrestrial volcanism and the Mid-Tertiary Orogeny. This regime extended from the Great Basin in Nevada and Utah, into Arizona-New
Mexico and to Sonora and is known as basin and range tectonics. It comprises gently tilted normal-fault blocks, marked by linear ranges and intervening valleys filled by thick piles of lacustrine and fluvial sediments.
At the deposit scale, both steep and low angle faulting is evident. Two main steep fault orientations are evident at Hermosa, a NNE-SSW to NE-SW trending structural zone and a conjugate NNW-SSE to NW-SE direction (Methven et al., 2018). A major steeply SW dipping NNW to NW trending structure, the Harshaw Creek Fault, lies on the western edge of the exposed wedge of Palaeozoic sedimentary rocks, separating them from the Triassic to Jurassic Older Volcanic Sequence/Mount Wrightson Formation and Cretaceous Bisbee Formation. To the north of the Hermosa deposits, it follows the boundary between the Triassic to Jurassic Older Volcanic Sequence and the Cretaceous Meadow Valley Andesites-Trachyandesites. A subparallel NNW-SSE trending, steeply east dipping zone of multiple faults truncates and offsets the same wedge of Palaeozoic sedimentary rocks, bounded to the east by the Older Volcanic Sequence. This zone controls much of the transgressive mineralisation in the Triassic to Cretaceous and Palaeozoic sedimentary rocks, including the steep NW-SE Trench Vein System and Alta Claims. It also occupies the longitudinal core of the Clark-Taylor deposit (see the geological map above) It has been variously known as the Hudson Fault Zone (Alta claim block), Trench-Josephine Fault Zone and January-Norton Fault Zone (Trench claim block). A member of the conjugate NE-SW fault set, a steeply north-dipping structure separates the Palaeozoic wedge from Proterozoic basement on the southern margin of the Hermosa property. As such it defines the southern limit of Palaeozoic sedimentary rocks in the immediate deposit area. A kilometre to the NW, a parallel fault separates the window of Palaeozoic rocks that is to the east of the Hudson Fault, from overlying Triassic to Jurassic volcanic rocks to its north.
Methven et al. (2018) note that exposures in the deposit area are commonly disrupted by irregular, discontinuous, complex structural zones that are characterised by rubbly, broken, brecciated and sheared features that do not typically, noticeably displace either lithologic contacts, alteration or mineralisation zones.
A low angle thrust fault, the 'Lower Thrust' identified by drilling, plays an important part in the geometry of the deposits on the Hermosa Property. This structure is assumed to be of Mesozoic or younger age. It is interpreted to be north to NE vergent, and emplaced three members of the Palaeozoic sequence, the Permian Concha, Scherrer and Epitaph formations, over the Triassic to Jurassic Older Volcanic Sequence creating a wedge of these volcanics between the Paleozoic sequence hosting the Taylor Sulphide mineralisation above and the Taylor Deeps below. It also juxtaposes the Older and Hardshell volcanic sequences, meaning it is Jurassic or younger. Its relationship to the Meadow Valley Trachyandesites is unknown. There is apparently no evidence that the thrust fault propagates through the entire Jurassic - Hardshell Volcanic Sequence, and it may have occurred during the deposition of Hardshell Volcanics. It appears to predate the steep faults which offset it (Methven et al., 2018).
A uniform light grey to tan colouration is evident in Jurassic rhyolitic rocks across much of the Hermosa property proximal to mineralisation. Distal to known mineralisation, the same rocks are generally shades of purple to maroon in outcrop. Locally, in otherwise unaltered rhyolite outcrops, small patches of fine-grained secondary K feldspar have been noted. The tan colouration has been interpreted (Methven et al., 2018) to reflect pervasive and moderately to strong potassic alteration, although primary volcanic and clastic textures are generally well preserved. The same authors regard this alteration to be a broad background upon which later alteration more directly associated with the mineralisation has been imposed.
Other alteration includes white kaolinite-sericite, which commonly selectively overprints clasts within the lithic tuff and breccia of the Hardshell volcanic sequence as veinlets and patches. Similarly, the fine-grained, tuffaceous, matrix to the lithic tuff, polymict breccia and lower rhyolite tuff are pervasively overprinted by very-fine, disseminated kaolinite-sericite. In both instances, primary textures are generally well preserved, and the rock remains hard and competent.
The stratabound manto mineralisation is embraced by an asymmetric alteration envelope of pervasive and strong silicification, previously referred to as 'jasperoid'. The best developed and most massive silicification is within rhyolite tuff in the immediate hanging wall of the mineralisation, commonly persisting for >10 m above the mineralisation. The primary minerals and textures in these volcanic rocks are completely replaced by grey, fine-grained quartz. Although rare, small patches or pods of vague relict volcanic texture have been noted in the hanging wall alteration zone. Where quartz-sulphide veins occur in the Jurassic volcanics, pervasive silicification of the host rock is seen to be associated with that vein-forming event also.
In the immediate footwall carbonate rocks, silicification is less complete and only penetrates a few metres below the volcanic-carbonate contact into the Concha limestone. The carbonate rocks of the Concha, Scherrer and Epitaph formations are weakly to moderately recrystallised and commonly bleached to a light grey colour. They also contain fine to coarse, irregular and discontinuous calcite veinlets. Fossils within these carbonates are normally well preserved along with fine primary sedimentary textures. In the northwestern part of the Hermosa property, increasingly pervasive and stronger recrystallisation of the carbonate rocks occurs, ultimately grading into diopside-wollastonite-rhodonite calc-silicate skarn with associated base metal sulphide mineralisation, as described below. Calcareous sandstone intervals contain fewer calcite veinlets, whilst quartzite only rarely host such veinlets.
Andesitic and trachyandesite lithologies, e.g., the Cretaceous Meadow Valley andesites, typically contain fine, thin, irregular and discontinuous calcite veinlets and may also contain finely-distributed groundmass calcite. Biotite, where present, is typically degraded with greenish chlorite selvages. Magnetite is occasionally noted and pyrite is not uncommon and alteration is regarded as being propylitic.
The Hermosa deposits comprise three main varieties of mineralisation, namely:
i). primary, concordant, carbonate replacement and skarn altered 'manto' in Permian sedimentary rocks, which includes the Taylor Sulphide and Taylor Deeps; ii). oxide facies (Clark deposit) lateral to and overprinting primary sulphide mantos of the previous variety; iii). both vein and manto mineralisation within the overlying Mesozoic volcanic sequences.
The Taylor Sulphide deposit occurs in the upper mineralised domain above the 'Lower Thrust', hosted by sections of the Permian Concha, Scherrer and Epitaph formations. Mineralisation is continuous for ~1350 m along strike, trending NW at ~310° with a width of ~455 m laterally beneath the eastern edge of the Hardshell claim, extending across the entire Alta claim to the southeastern edge of the Trench claim block. Mineralisation thickness varies with the host unit, with ~60 m in the Concha, ~18 m in the Scherrer and ~90 m in the Epitaph formations.
The Taylor Deeps lies below the 'Lower Thrust', and is characterised by a calc-silicate mineralogy. It is hosted within the Permian Concha Formation carbonate sedimentary rocks, below the thrusted base of the Older Volcanics, at a depth of ~1035 m below surface. It averages ~23 m in thickness, with a strike length of 2100 m at a trend of ~310° to the NW, and a width of up to 790 m.
Sulphide mineralisation at the Taylor deposits includes zones of massive replacement of carbonate by galena, sphalerite, chalcopyrite and pyrite up to 6 m, and occasionally to as much as 30 m thick, with associated calc-silicate alteration. This alteration becomes more skarn rich, particularly in the northwestern part of the Hermosa property. The calc-silicate-skarn alteration occurs as patches and more extensive hard, massive, wholesale replacement of carbonate by generally white to pink, very fine-grained to aphanitic, wollastonite-diopside and rhodonite. Significant sections of this skarn contain sparse coarse-grained, radiating crystal aggregates up to 2 cm across, commonly accompanied by coarse-grained, euhedral to subhedral galena, sphalerite, chalcopyrite and pyrite. Light green, massive, coarse-grained garnet with abundant sulphides as disseminations, pods, masses and interstitial replacements are sparsely distributed, deep within the Epitaph Formation in the Taylor Sulphide deposit, where they are directly related to intrusive dykes and sills.
Representative intersections of Taylor calc-silicate-sulphide mineralisation were found to be composed of 2.2 to 32.6, averaging 8.5% sphalerite; 1.2 to 31.1, averaging 8.5% galena; with Cu-sulphides that mainly included trace to 1.8, averaging 0.2% chalcopyrite, an average of 0.1% tetrahedrite, with traces of chalcocite, bornite and other Cu-sulphides. The principal gangue sulphide was pyrite that ranged from 0.1 to 9.6 and averaged 4.6%. Other minerals included 24.4% silica/quartz, 10.7% rhodonite, 10.6% rhodochrosite and 8.3% calcite, accompanied by a number of Zn-bearing silicates.
The Clark (or Central) deposit is developed as an overprint to the Taylor Sulphide mineralisation within the Permian Concha Formation, but also extends into the overlying rhyolites of the Jurassic Hardshell Volcanic Sequence in the immediate hanging wall. It is composed of oxide manganese, zinc and silver mineralisation. The oxidised rhyolites overlying the carbonate units contain irregular patches and zones of veinlet-controlled hematite-limonite and sooty Mn-oxide with accessory zinc and silver mineralisation. Manto-style mineralisation in rocks of rhyolitic composition is dominated by black, sooty cryptomelane, with or without yellowish-orange secondary lead-oxides, and with a quartz-dominant gangue mineralogy. Oxide manto-style mineralisation in the carbonate rocks below does not typically contain lead-oxides. Strong, pervasive grey, silicification is also developed and calcite occurs as veinlets, vugs and fracture fill. Drill core intersections containing rhodochrosite and pink calcite are common, whilst rarer sections of hard pinkish rhodonite-bustamite are also observed.
Broadly stratabound volcanic hosted lenses of both sulphide and oxide mineralisation also occur throughout the Hardshell Volcanic Sequence, and are known collectively as the 'Hardshell Zone'. They are intersected in drill holes from surface testing the underlying carbonate hosted manto mineralisation and appear to be stacked above that zone, although this may be an artefact of the drilling pattern. This zone was the basis of historic production from the Hardshell Incline mine. The principal mineralised horizon in this zone is ore is composed of a 3 to 30 m thick 'Rhyolite Polymict Breccia', a minor proportion which was composed of often large (to 3 m diameter) sedimentary carbonate clasts. This unit has been subjected to partial to massive Mn-oxide replacement in the southeast, and partial to massive Pb-Zn sulphide replacement mineralisation in the northwest beneath the Alta and Trench claims. The sulphides principally replace sedimentary carbonate clasts within the breccia.
Vein-hosted sulphide mineralisation principally occurs in northwest as 310°, trending structural zones, that dip steeply at 75 to 85°NE, but are also found with strikes that include NNW, WNW and NE. Vein thicknesses vary from 0.5 up to 2 m, occurring as single veins or vein zones up to 6 m thick with strike lengths of up to 1500 m, although the main Trench veins form an anastomosing composite zone with a strike of 2.6 km, distributed over a width of between 120 and 250 m. These veins are composed of white, massive quartz with open-space, growth-zoned quartz crystals, containing coarse grained sulphides pyrite, galena and sphalerite. Such quartz–sulphide veins have been encountered in all stratigraphic formations on the Hermosa Property and are interpreted to be related to, and coeval with, both the carbonate replacement manto mineralisation in the Palaeozoic sequence, the 'Hardshell Zone' mineralisation in Jurassic volcanic rocks and the veins exploited by ASARCO on the Trench Claims hosted by the Cretaceous Meadow Valley Andesite to Trachyandesite.
District Scale Alteration and Mineralisation
The Hermosa deposits are located immediately to the ESE of the greater Sunnyside porphyry copper mineralised system. The focus of this system, is a deep sub-economic porphyry Cu-Mo deposit, concealed below >1100 m of variably mineralised overburden. The mineralisation of this deposit is an inverted shell (convex up) ~250 to 300 m thick, with a diameter of ~900 m, vertical extent of ~800 m, hosted in the apex of a potassic altered, ~60 to 59 Ma quartz-feldspar porphyry. A non-JORC and non-NI 43-101 compliant resource estimate of ~1.5 Gt @ 0.33% Cu, 0.011% Mo, 5 g/t Ag at a 0.2% Cu cut-off, with higher grade internal zones has been estimated for this deposit (Graybeal et al., 2007, Vikre et al., 2014).
The porphyry deposit is overlain by a thick and areally much more extensive advanced argillic alteration lithocap that persists to the surface, and is characterised by strong silicification grading to a quartzite, with variable alunite, kaolinite, diaspore, zunyite, tourmaline and rare pyrophyllite. This lithocap alteration and associated mineralisation is concentrated in the more permeable Mesozoic volcanic and volcaniclastic sequences. The central silica rich core of the lithocap at surface grades outward and downward into a pervasive phyllic quartz-sericite-pyrite zone which separates it from the potassic alteration of the deep porphyry deposit. All of these alteration zones are surrounded at surface by a large, NNW-SSE elongated ~8 x 3.5 km zone of pyritisation, broadly coincident with the outer pervasive phyllic alteration envelope. The interval extending from over the western edge of the deep porphyry deposit to the eastern margin of the zone of pyritisation is occupied by a mass of enargite-chalcocite-tennantite mineralisation which constitutes a weak ~59 to 58 Ma resource of ~800 Mt @ 0.175% Cu, 7 g/t Ag (Graybeal 1996, Vikre et al., 2014).
The Hermosa deposits have been traced WNW to the rim of the exposed pyrite/pervasive phyllic halo (see the geological map above). The strongly siliceous alteration of the Sunnyside lithocap appears to plunge below the Cretaceous volcanic cover to connect with the similar silicification that is closely associated with manto Zn-Ag-Pb mineralisation within both the Hardshell Volcanic sequence, and at the basal contact of the Older Volcanic Sequence with the underlying, reactive, Permian host carbonates of the Taylor and Clark deposits. The overlying Cretaceous Meadow Valley Trachyandesite have been propylitised and carry Cu, Pb, Zn, Ag and Au veins as at the Trench mine. According to Graybeal (1996) a wing of phyllic veining entends to the SE from the margin of the pervasive phyllic zone to overlap the Hermosa deposits.
In addition to the Zn-Ag-Pb mineralisation at Hermosa, which becomes increasingly skarn altered to the NW, similar mineralisation has been encountered in drilling at depths of 1.2 to 1.6 km in skarn along the southeastern margin of the Sunnyside deep porphyry Cu-Mo deposit. This mineralisation is hosted within skarn altered Palaeozoic carbonate rocks in the footwall of the Older Volcanic Sequence, with intersections of 15 to 30 m of ~1% Cu, 3% Pb, 10% Zn and 300 g/t Ag reported (Vikre et al., 2014) regard the Hermosa deposits to be a distal arm of the Sunnyside mineralised system (Graybeal 1996, Vikre et al., 2014).
Graybeal (1996) and Vikre et al. (2014) regard the Hermosa deposits to be a distal arm of the Sunnyside mineralised system. The observations outlined above suggest the Sunnyside and Hermosa deposits are parts of an evolving district scale mineralised system. The first phase involved the development of the Sunnyside deep porphyry Cu-Mo deposit within the quartz-feldspar porphyry intrusive complex, the result of a number of co-spatial, juxtaposed intrusive and hydrothermal fluid events sourced from a deep parental magma chamber. The last of the intrusions related to this first stage propagated to shallower depths and erupted in a phreatomagmatic explosion, forming a vent complex filled by the volcaniclastic tuff unit. This event led to decompression of the porphyry system, boiling and the separation of the vapour phase, as indicated by fluid inclusions within the quartz-feldspar porphyry and volcaniclastic tuff at depths shallower than 600 m (Graybeal 1996) and a change in pH. These changes and opening of the conduit to shallower levels allowed the widespread distribution through permeable volcanic rocks of differentiated fluids from the apex of the large, deep parental magma chamber. The low pH vapours and following fluids produced the advanced argillic and phyllic lithocap. These fluids/vapours were distributed through more permeable facies of the Older and Hardshell Volcaniclastic sequences to be deposited as epithermal veins in open fractures in more brittle lithologies, and by reacting with carbonates adjacent to these permeable conduits. Faults cutting the sequence aided access of fluids to the carbonates, e.g., the fault that follows the axis of the Taylor and Clark deposits (see the geological map above).
These deposits are part of larger district scale pattern of porphyry, vein and breccia deposits detailed in Vikre et al. (2014).
Pre-mining JORC compliant Mineral Resources published include (South32 Limited ASX Release June 2019) at 31 May, 2019:
Measured Mineral Resources - 21 Mt @ 4.07% Zn, 3.61% Pb, 51 g/t Ag;
Indicated Mineral Resources - 91 Mt @ 3.27% Zn, 3.73% Pb, 74 g/t Ag;
Inferred Mineral Resources - 43 Mt @ 3.32% Zn, 3.56% Pb, 67 g/t Ag;
TOTAL Mineral Resources - 155 Mt @ 3.39% Zn, 3.67% Pb, 69 g/t Ag;
Underground Sulphide TOTAL Mineral Resources - 149 Mt @ 3.32% Zn, 3.66% Pb, 70 g/t Ag;
Underground transition TOTAL Mineral Resources - 6.2 Mt @ 5.22% Zn, 3.82% Pb, 57 g/t Ag.
Pre-mining JORC compliant Mineral Resources published include (South32 Limited ASX Release May 2020) at 12 May, 2020:
Indicated Mineral Resources - 33 Mt @ 2.49% Zn, 9.39% Mn, 56 g/t Ag;
Inferred Mineral Resources - 22 Mt @ 2.04% Zn, 8.64% Mn, 110 g/t Ag;
TOTAL Mineral Resources - 55 Mt @ 2.31% Zn, 9.08% Mn, 78 g/t Ag; all of which are underground oxide resources.
This summary is largely drawn from "Methven, G., Nussipakynova, D., Bloom, L., Jin, Q., Kaye, C., Smith, R.M., Kottmeier, C., Bartlett, D. and Christenson, E., - Hermosa Property Mineral Resource and Taylor Deposit, Santa Cruz County, Arizona, USA, PEA update; An NI 43-101 Technical Report prepared by AMC Mining Consultants (Canada) Ltd. for Arizona Mining Inc. 245p., but also includes lesser details from earlier technical reports available from Sedar.com from Nov. 2016; March, 2016; Jan. 2014 and Oct. 2010.
The most recent source geological information used to prepare this summary was dated: 2018.
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.
Graybeal, F.T., 1996 - Sunnyside A Vertically-Preserved Porphyry Copper System, Patagonia Mountains, Arizona: in SEG Newsletter No.26, p. 1, 10-14.|
Graybeal, F.T., Moyer, L.A., Vikre, P.G., Dunlap, P. and Wallis, J.C., 2015 - Geologic map of the Patagonia Mountains, Santa Cruz County, Arizona: in U.S. Geological Survey, Open-File Report 2015-1023, 10p.|
Vikre, P.G., Graybeal, F.T., Fleck, R.J., Batron, M.D. and Seedorff, E., 2014 - Succession of Laramide Magmatic and Magmatic-Hydrothermal Events in the Patagonia Mountains, Santa Cruz County, Arizona: in Econ. Geol. v.109, pp. 1667-1704.|
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