Cerro Negro - Eureka, Mariana, San Marcos, Bajo Negro, Vein Zone
Santa Cruz, Argentina
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The Cerro Negro low to intermediate sulphidation epithermal gold-silver deposits are located in Santa Cruz Province, southern Patagonia, Argentina, ~60 km SE of Perito Moreno and ~240 km SW to WSW of Comodoro Rivadavia (#Location: Vein Zone - 46° 53' 24"S, 70° 11' 57"W).
The Cerro Negro Operation includes six major mineralised zones, the Bajo Negro, Eureka, Mariana Central, Mariana Norte and San Marcos zones that are to be mined underground, and the Vein Zone, to be exploited by open pit. These vein systems are concentrated in two areas. Eureka, Mariana Central, Mariana Norte and San Marcos occur progressively from SW to NE over an elongated 7 x 4 km area to the NW, whilst Bajo Negro and the Vein Zone are 2.5 km apart, ~10 km to the SE.
Exploration in the Cerro Negro district has been conducted by a number of companies since the early 1990s. An Argentine prospector first identified the potential for gold mineralisation in 1992, in what is called 'Silica Cap', 1 km south of the Vein Zone at Cerro Negro. Minera Newcrest Argentina S.A. conducted
a reconnaissance exploration program over sections of the Deseado Massif in 1993, resulting in discovery of the Eureka, Mariana, El Retiro and Las Margaritas mineralisation, as well as what became known as the Vein Zone. The company also entered into an agreement with the prospector who had recognised the 'Silica Cap' zone. After further exploration, Newcrest decided the mineralisation did not meet their target criteria and partially withdrew from the area. Subsequently, MIM Argentina Exploraciones secured the ground dropped by Newcrest, but later entered into a joint venture with Newcrest to explore both their holdings. Newcrest withdrew from the JV in 1999. In 2000, Oroplata S.A., an Argentine company owned by Oroplata Pty Ltd entered into an option with MIM and in 2003 signed an agreement to acquire 49% of the project. In late 2003, Andean Resources Limited acquired MIM's 51% holding and in 2004 completed a takeover of Oroplato. Exploration continued to outline significant mineralisation and in 2008 a pre-feasibility study was completed on the Eureka and Vein Zones, and a feasibility study was conducted in 2010 on the Eureka, Vein and Bajo Negro Zones. In 2010, Goldcorp Inc. acquired Andean Resources. Declines was commenced at Eureka in May 2010, at Mariana Central in March 2012, and at Mariana Norte in May 2012. A small open pit was started at Eureka in 2013 and a processing plant was commissioned in July 2014, with commercial production on 1 January 2015. Production during 2015 totalled 1.63 Mt @ 13.72 g/t Au, 204.2 g/t Ag for 20.5 t Au and 258 t Ag.
The Cerro Negro Au-Ag veins are situated near the northwestern margin of the extensive Deseado Massif that stretches across southern Argentina. This massif is largely occupied by rocks of the Jurassic Chon Aike large igneous province, related to a late Triassic to late Cretaceous (230-65 Ma) extensional phase that culminated in the opening of the South Atlantic Ocean. The massif passes westward into the Mesozoic to Cenozoic subduction related Andean arc, and is flanked by the post-Jurassic subsiding Golfo de San Jorge and Austral sedimentary basins to the north and south, respectively. The massif is underlain by the Patagonian Terrane, an early Palaeozoic to Permian allochthonous terrane that collided with Gondwana in the early Permian.
The Chon Aike large igneous province magmatic activity, represented by the Jurassic Bahia Laura Volcanic Complex, commenced in the early Jurassic, with the eruption of pyroclastic and epiclastic volcanic rocks. Basaltic to basaltic andesite and dacitic to rhyolitic volcanism continued through the mid- to late Jurassic, culminating in the deposition of epiclastic sediments in the early Cretaceous. Mid- to late Jurassic volcanism in this complex is divided into the basaltic to basaltic-andesites of the Bajo Pobre Formation (e.g., Páez et al., 2010), overlain by predominantly dacitic to rhyolitic ignimbrites, tuffs and lesser lavas of the Chon Aike and La Matilde Formations. A second andesitic pulse occurs towards the top of the Chon Aike Formation. A hiatus over the Deseado Massif from the Lower to Late Cretaceous is represented by a regional unconformity, until basaltic volcanism commenced in the Late Cretaceous and continued intermittently through the Cenozoic. Over the same period, fluvial to lacustrine and shallow marine sedimentation was continuous from the Late Jurassic to Late Tertiary in the Golfo de San Jorge and Austral basins. All of these rocks are overlain by extensive Pleistocene fluvial gravel terraces.
The location and regional setting and geology of the Deseado Massif is described in more detail in the Southern Andes and Patagonia record.
The western section of the Cerro Negro district contains a ~500 m thick sequence of Jurassic volcanic rocks that uncomfortably overlie a Palaeozoic basement of low-grade metasedimentary and granitic rocks. The latter are not exposed in the area, only having been encountered in drilling. The distribution of the volcanic sequence and related sub-volcanic intrusions is influenced by major WNW to NNW and NNE to NE, and subordinate east-west faults. These faults acted both as conduits for sub-volcanic intrusions feeding the volcanism and as structures that define the margins of grabens filled by the volcanic rocks. Gold-silver vein systems at Vein Zone, Eureka, Bajo Negro, Mariana Sur and Mariana Central strike NW to WNW, although Vein Zone and Eureka also have subordinate east-trending segments. The San Marcos mineralisation is hosted by an east-trending vein that splays off a NW-trending fault. The Mariana Norte vein trends approximately east-west.
The Au-Ag deposits and prospects known in the Cerro Negro district occur within and to the east of the Eureka Volcanic-Subvolcanic Complex that is flanked and overlain by a series of rhyolite domes. The eruptive products of the upper rhyolite domes form an ignimbrite apron that post-dates the mineralisation and is extensively developed within grabens to the north and south of the volcanic-subvolcanic complex. These post-mineral ignimbrites protected the epithermal systems, as well as the lacustrine sediments, travertine and sinter deposited at the Late Jurassic paleo-surface from erosion. Older ignimbrites that lie east of the volcanic-subvolcanic complex host mineralisation at Bajo Negro and Vein Zone.
Vein mineralogies are influenced by their location relative to the Eureka Volcanic-Subvolcanic Complex. Veins within the Complex (e.g., Eureka, San Marcos and the Marianas) contain significant silver as well as gold. The Eureka veins contain abundant adularia and ginguro-style sulphides (i.e., thin, silvery-black bands of fine grained sulphides and electrum). In contrast, veins outside the dome hosted by the Cerro Negro Ignimbrite (e.g., Bajo Negro and Vein Zone) contain lower silver grades, coarse pyrite rather than ginguro sulphides, and lack macroscopic adularia or carbonate in the gangue.
The Middle to Upper Jurassic volcano-sedimentary sequence can be summarised as follows, from the base (after Vidal et al., 2016; Tripp et al., 2015):
• Mariana Basaltic Andesite - composed of porphyritic and trachytic textured hornblende-pyroxene-magnetite basaltic andesite lava flows and lahar agglomerates. These rocks are the main host to the Marianas-San Marcos vein system and are characterised by porphyritic texture with plagioclase and pyroxene phenocrysts and abundant vesicles.
• Mariana Rhyolite - commencing with surge deposits that are overlain by rhyolitic flow-domes and high grade ignimbrites which are phenocryst poor glass altered to quartz. These outcrop at Eureka, Mariana, Eureka Norte and Eureka Este.
• Lower Eureka Ignimbrite - comprising welded hornblende-pyroxene dacitic ignimbrites with 3% embayed quartz crystals and basaltic andesite fragments. Lahars with organic rich layers found in both the basal and uppermost sections of the succession.
• Eureka Andesite (or Medium Andesite of Vidal et al., 2016) - composed of porphyritic (phenocrysts to 4 mm) plagioclase, pyroxene andesitic domes, lava flows and lahar agglomerates. These have been dated at 159±2.1 Ma (U-Pb zircon; Lopez, 2006).
• Dacite and Trachyandesite intrusions and domes - comprising K feldspar, hornblende and biotite phenocryst-rich dacite, and an apatite-ilmenite bearing plagioclase and pyroxene phenocryst-rich trachyandesite, dated at 157±1.7 Ma and 156±1.2 Ma respectively (U-Pb; Lopez, 2006). These intrude all of the volcanic units described above, and occur as domes above the Eureka Andesite and within the Upper Eureka Ignimbrite.
• Upper Eureka Ignimbrite which marks the top of the lower section of the volcaniclastic sequence. It is characterised by the aphanitic trachyandesitic lava flows and domes of the Upper Andesite unit, and the porphyritic dacitic domes, sub-volcanic intrusions and widespread moderately welded dacitic pyroclastic flows within the Upper Eureka Ignimbrite. The latter has a high-K to alkaline composition and comprises a hornblende-biotite-apatite dacitic ignimbrite with 3 to 5% strongly embayed quartz.
• Mariana Basin hot springs sequence fluvio-lacustrine deposits developed during the waning stages of acidic volcanism, occurring as mounds and graben fill above, and partially interfingering with the Upper Eureka Ignimbrite. This sequence includes chalcedonic and travertine mounds, opalite layers and waterlain sediments containing pyritic white clay and advanced argillic altered tuff. It is part of a succession that includes polymictic brown muddy matrix volcaniclastic rocks that are interbedded with brown mudstone, tuffaceous silts and dacitic white tuffs found within graben structures.
• Eureka Rhyolite - to the south, this unit interfingers with the Mariana Basin hot springs sequence and is composed of rhyolitic to dacitic ignimbrites. It is overlain by a complex of rhyolite domes and associated distal tuffs and welded rhyolitic ignimbrite, followed by regional rhyolitic ignimbrites.
• Matilde Formation - that unconformably overlies all of the preceding units, and predominantly comprises fine-grained biotite-rhyolitic tuff and tuffaceous siltstone.
• Eocene to Pliocene - andesitic basalt lava flows and younger superficial cover.
Vidal et al. (2016) divide this sequence, which represents the Bahía Laura Volcanic Complex of Páez et al. (2010), into an Upper and Lower section on either side of the top of the Eureka Andesite. They regard the Lower Section to equate with the Bajo Pobre Formation (e.g., Páez et al., 2010), and the Mariana Basin hot springs sequence and Eureka Rhyolite of the Upper Section to correlate with the Chon Aike Formation. All of the mineralisation is hosted in the Lower section below the Mariana Basin hot springs sequence. However, Tripp et al. (2016) interpret the Marian Basaltic Andesite to solely represent the Bajo Pobre Formation, with the Chon Aike Formation beginning at the base of the Lower Eureka Ignimbrite.
Each of the main vein systems may be summarised as follows:
The Eureka vein system, which is located on the north western margin of the Cerro Negro field, can traced over a length of 4.6 km, between cover rocks to the NW and hot-spring deposits to the SE. The vein system strikes NW to east-west and dips SW to south. The entire mineralised zone, including both stockwork and veining, can reach 100 m in width. However, within this area, the higher-grade mineralisation is confined to a strike length of ~1500 m, with economic widths locally as much as 27.8 m, but averaging ~8.4 m. The host rocks are typically intrusive andesite in the hanging wall and a sequence of andesitic rocks and more felsic porphyries and ignimbrites in the footwall. The Eureka Vein System has been divided into three main segments:
• Southeast segment (the 'Eureka Southeast Vein' or 'Eureka Southeast Extension'), which is 3.7 km long, striking NW, with discontinuous vein outcrops, bounded by the Cañadon del Puma gorge to the NW, and by outcrops of sinter and related geothermal discharge deposits to the SE.
• Central segment - a 450 m interval in which the veins strike east-west, and includes the Eureka Main, and other smaller veins. This segment is bounded to the east by the Cañadon del Puma gorge, and to the west by the easternmost exposures of the Eureka West vein segment;
• Western segment which comprises a 700 m long, continuous, NW striking vein system, mostly concealed by cover rocks. It consists of two main splits, known as the West Vein and First Footwall Vein. A hanging wall vein and a further footwall vein NE of the first footwall vein have also been identified.
Vein textures include colloform and crustiform banding, cockade and manganese-iron oxide matrix breccias. At deeper levels, particularly in the Main Vein, delicate alternating colloform bands of quartz and adularia are developed, and bonanza gold-silver grades are associated with dark, fine-grained ginguro sulphide bands. Native gold ranges from 10 to 40 µm, and locally 100 to 200 µm in width. Native silver and electrum grains are in the range 50 to 200 µm. Both native gold and native silver are abundant in dark quartz veinlets or on their margins. Possible post-mineral hypogene or deep supergene oxidation has remobilised silver.
Mariana Central and Mariana Norte
Mineralisation at Mariana Sur, Mariana Central, Mariana Norte and San Marcos is hosted by fine-grained andesitic volcanic rocks correlated with the Mariana Basaltic Andesite, the oldest exposed unit in the Cerro Negro vein field. The veining and immediate host sequence is buried beneath thick rubbly collapse breccia deposits that are intercalated with felsic ignimbrite at the base of the Eureka Rhyolite. The breccia is polymictic, matrix supported, and is composed of angular to sub-angular, variably-sized clasts up to tens of metres in diameter. The matrix is mostly comminuted rock flour material with abundant greenish tinted clays (smectite) and late thin calcite and gypsum veinlets. Clasts are composed of hydrothermally altered host rocks, silicified breccias, silicified travertine deposits and mineralised vein clasts that are distributed above and close to the hidden veins. High-grade clasts in these breccias were the key to the discovery of the blind Marianas veins (Shatwell et al., 2011). The size and abundance of banded vein fragments increases toward the buried mineralised veins. The presence of hydrothermal breccias cutting part of the collapse breccia indicate that hydrothermal activity was still active while it was being formed, although clasts of mineralised vein material within collapse breccia indicate it was dominantly formed during the waning stages of hydrothermal activity. A rhyolite flow-dome of unknown age is exposed a few hundred metres west of the Mariana veins.
The Mariana veins were predominantly formed within dilation zones on major NW-trending normal, mainly dip-slip faults and/or splays. The major faults are interpreted to be reactivated older structures that persist into the pre-Jurassic basement. Continued syn- and post-mineral movement on these faults resulted in footwall collapse and deposition of the post-mineral breccias over the veins within these same structures. The faults at Mariana Norte and Mariana Central form part of a half-graben margin, with the stratigraphic succession dropped down stepwise to the north.
Mariana Central is ~3 km ENE of Eureka. The Mariana Norte and Mariana Central veins strike at 280 and 305° respectively and are 250 m to 500 m apart. Both dip to the north, at angles of 60 and 65° respectively.
Mariana Central comprises i). the main vein; ii). a hanging wall split that separates from the main vein near the east end of the deposit and then rejoins the main vein 300 m farther to the WNW; iii). a separate, roughly parallel, secondary hanging wall vein that occurs ~100 m into the hanging wall of the main and hanging wall split veins; and iv). a third, roughly parallel, hanging wall vein ~100 m from the main, interior to the hanging wall split vein. All of the defined veins have associated small, discontinuous subparallel veins.
The main vein is 800 m long by 300 m down-dip and up to 23 m thick, but averaging >7.1 m. Southeasterly extensions, the Mariana Southeast and Emilia veins, extend the length of the known mineralisation by >1 km. Mariana Central is the highest Au-Ag grade vein in the district.
Mariana Norte comprises a main vein, with an adjacent, discontinuous, high-grade subparallel footwall vein. A hanging wall split separates from the main vein near the eastern end of the deposit, diverging at an angle of about 20°, dipping at >80°N. The main vein is 700 m long by 400 m down dip, and up to 26.9 m thick, averaging 7.4 m. The Mariana Norte extension expands the known length of the system by several hundred metres.
San Marcos is ~1.5 km ENE of Mariana Norte. South of the east-west trending San Marcos vein system, a WNW trending fault/breccia separates fine-grained andesite of the Mariana Basaltic Andesite (the basal member of the Eureka Volcanic-Subvolcanic Complex) to the NE, from rhyodacitic ignimbrite of the La Matilde Formation to the SW. This fault is reflected by a strong magnetic lineament that can be traced for at least 11 km. The fine-grained andesite is the host to all known mineralisation at San Marcos. Mineralisation at San Marcos occurs as a braided vein system, dominated by two primary and two separate sub-parallel veins, and a hanging wall split. The two primary veins, the main hanging wall and main footwall veins, are more continuous and predictable than the subsidiary veins. They strike east-west and persist over a strike length of 750 m. These veins, and the subsidiary hanging wall and footwall veins, are typically vertical, though in places the dip is as low as 80°. The hanging wall vein split diverges from the primary vein at an angle of ~40°, dipping near vertically, before rolling over and dipping SSW, opposite to the main vein. While this hanging wall split forms a relatively well-defined structure, it carries very little gold. Weakly-mineralised quartz-vein stockworks occur in the adjacent wallrocks to the main veins. The main hanging wall vein, which is on the north side, averages 6.5 m thick and has a maximum thickness of 20.4 m. The main footwall vein is up to 9.8 m thick, averaging 4.7 m. Mineralisation consists of white quartz with abundant coarsely crystalline pyrite, vein breccias and some black ginguro banding.
The Vein Zone deposit is located ~10 km SE of San Marcos and comprises a complex north-south to WNW arcuate system of discrete quartz veins, sheeted and stockwork veins and breccia zones, hosted by a sequence of welded rhyodacitic ignimbrites. The zone is ~500 m long and has been traced over a 400 m vertical interval, with a thickness of up to 80 m, excluding the footwall vein. The veins have easterly, northwesterly, or northerly strikes and steep to sub-vertical dips. The overall deposit dips at ~60°NE. Veining is controlled by a complex 80°N to NE dipping fault zone that forms the boundary between strongly and moderately welded ignimbrites. The intersection of NW and east trending veins may have been important in localising mineralised shoots.
Mineralisation is oxidised, interpreted to be the result of i). hypogene alteration by a low-pH hydrothermal fluid and ii). post-mineral oxidation due to near-surface weathering. Gold is associated with oxidised pyrite and manganese oxide along with hematite-goethite, minor sphalerite, kaolinite, illite and adularia. Arsenic, manganese and barium are locally anomalous. Platy quartz that is a pseudomorph of carbonate, colloform banding, and open or clay-filled vugs accompanies the gold. Mineralisation occurs within an extensive envelope of kaolinitic alteration that changes sharply to illite alteration in the footwall.
The Bajo Negro vein is located ~2 km SW of the Vein Zone and is hosted by a relatively uniform sequence of weakly to moderately welded dacitic ignimbrites. It has a strike length of almost 1200 m, a vertical extent of as much as 300 m, and a maximum thickness of 16 m, averaging 7.4 m. The deposit is essentially a single structure striking at ~330° and dipping at 65 to 75°NE in its central half, although in jogs the strike changes to ~310° while the dip flattens slightly in its northwestern and southeastern ends. Deeper drilling suggests the vein is on a contact, interpreted to be a normal fault, between a hanging wall ignimbrite and footwall lithic tuff or less-welded ignimbrite. The southwestern termination of the vein is steep and inferred to be faulted, whilst the northwestern extremity is cut by a post-mineral breccia, whose northeastern contact dips to the SW. This breccia body includes multiple styles of brecciation with numerous clast types, including mineralised vein quartz, milled-matrix breccia, and clast-supported hydrothermal breccia. Most surface exposures of the breccia are silicified. Drill hole intersections of vein quartz within the breccia are variously interpreted as clasts or as in situ veins, suggesting the breccia is a late syn-mineralisation event. However, the ignimbrite-breccia contact is treated as a hard upper limit to mineralisation in resource estimation.
Wall rocks are altered to a quartz-sericite-pyrite assemblage (Guido and Permuy, 2009), commonly with adularia, both as veinlets and replacement. Pyrite is generally oxidised, with features suggesting kaolinite and hematite found throughout may be both supergene and hypogene in origin (Corbett, 2009).
The vein is filled by chalcedonic to crystalline quartz plus well-crystallised pyrite or (more commonly) iron oxide after pyrite. Native gold, some of which is likely supergene, is commonly visible. Bladed quartz replacing carbonate is present in most drill intersections of the vein. Much of the vein is brecciated and cemented with jasperoid (silica plus iron oxide). Small amounts of sphalerite, galena and chalcopyrite, as well as pyrite, are associated with higher gold grades, while chalcopyrite is also present in lower-grade samples. Electrum is common, as is native gold, in samples with grades of >10 g/t Au. Gold particles vary from 5 to 50 µm, while electrum is somewhat coarser grained and mostly in the range 20 to 60 µm (in one case, 150 µm), while native silver has a wide size range from <5 to 150 µm. Vein alteration minerals include kaolinite, illite and smectite, with variable amounts of barite and alunite. Adularia is largely replaced by quartz and clays.
The following observations and interpretations are derived from a study of the Mariana Central and Mariana Norte veins (Vidal et al., 2016), which represent ∼50% of the gold-silver resources in the Cerro Negro district. These veins have a formation history that has been divided into ten stages with crosscutting relationships that have been grouped into four main episodes:
• Episode 1 - a low volume, metal-rich initial episode, which is composed of four stages:
Stage 1 - early tabular adularia crystals intergrown with coarse quartz, followed by colloform-crustiform banded fine-grained quartz-adularia with clays and sulphide-rich bands and patches carrying up to 1000 ppm Au and 10 000 ppm Ag over widths of <1 m. Quartz appears to have been partially to almost completely recrystallised from chalcedony or amorphous silica. Massive and lattice texture quartz are intergrown, the latter reflecting replacement of platy calcite. Quartz bands alternate with rhombic adularia and greenish chlorite-smectite clay-rich bands, where later quartz is typified by coarser crystals of amethyst. Clays of the same composition, accompanied by kaolinite, also crosscut the vein banding and occur as probable late breccias. Early ore minerals include pyrite, chalcopyrite, Fe-poor sphalerite and minor galena, followed by acanthite and polybasite-pearceite with up to 4.7% Se. Minor associated tetrahedrite and pyrargyrite also occur in this stage. Late ore forming minerals include massive chalcopyrite with electrum which has a mean Au content of 60%.
Stage 2 - is characterised by delicate colloform banding of chalcedony (partially recrystallised to quartz) and adularia with quartz lattice texture after platy calcite and replacement texture after pseudoacicular minerals. Clay-rich bands are rarer than in stage 1, and they are composed of chlorite-smectite, illite and kaolinite. Ore minerals are also less abundant than in stage 1, and include pyrite, chalcopyrite, sphalerite (with chalcopyrite disease) and minor acanthite and galena (with inclusions of hessite), Ag-sulphides and sulphosalts. Gold is dispersed, occurring as 70% fineness electrum associated with chalcopyrite, and as inclusions in pyrite or intergrown between quartz crystals.
Stage 3 - has a characteristic brecciated texture in which distinct tabular clasts of fine-grained colloform-banded chalcedony containing abundant sulphides are set in a cement of fine-grained (mosaic) quartz with abundant cavities that are filled with massive to coarse-grained quartz and clay minerals. Ore minerals are dispersed in the matrix, within tabular clasts and as late precipitates filling cavities. They include pyrite with other sulphides and minor electrum inclusions, sphalerite, acanthite, polybasite-pearceite, pyrargyrite, and free electrum.
Stage 4 - occurs as thin barren veinlets of quartz, adularia and albite. Tabular to subrhombic adularia crystals grow outward from the selvage and are partially overgrown by later albite. Toward the centre of the veins, adularia crystals are intergrown with coarse-grained quartz and late calcite.
• Episode 2 - characterised by voluminous banded quartz emplacement in all structures, accounting for 70 to 75% of the vein material, with only minor mineralisation. It has lower precious metal contents compared to episode 1, and has been subdivided into three stages, continuing as:
Stage 5 - that consists of quartz with lattice textures in which cavities are filled with late amethyst, marcasite, arsenopyrite and minor kaolinite.
Stage 6 - a breccia, composed of angular to sub-angular clasts of earlier vein fill, and a coarse-grained to massive quartz and late amethyst matrix. Ore minerals include coarse-grained pyrite containing acanthite, sphalerite and some electrum inclusions, and chalcopyrite.
Stage 7 - which is broadly distributed and characterised by veins and veinlets filled by fine-grained rhombic adularia followed by colloform chalcedony banding (with feathery and plumose textures), coarse-grained quartz, and late amethyst.
• Episode 3 - a barren waning phase episode, occurring in a further two stages, both of which have low Au-Ag contents.
Stage 8 - typically present in the northern veins (Mariana Norte and San Marcos) where it is characterised by quartz with colloform-crustiform banding and breccias composed of fine-grained, hematite-stained quartz (after chalcedony and amethyst), accompanied by illite and minor apatite and titanite. Ore minerals are disseminated in the quartz bands, and associated with massive pyrite (with electrum inclusions), sphalerite, and minor marcasite.
Stage 9 - is characterised by widely distributed barren veinlets, veins and breccias infilled and cemented by coarsely crystalline, rhombic calcite, with some quartz and amethyst.
• Episode 4 - representing a silver-rich late tectonic-hydrothermal event. This episode is restricted to NNW and east-west aligned structures and comprises:
Stage 10 - which consists of discontinuous bodies of matrix- to clast-supported breccias set in a groundmass of chalcedony, illite±chlorite and iron oxide, with minor apatite. These breccias are confined to the vein structures and appear to be the result of late fault movement. Clasts, which are derived from earlier vein filling, have a diverse range of size and shape. The breccias locally exhibit millimetre-scale deformation banding and cataclastic textures whereby clast size progressively diminishes. Ore minerals are common where these breccias intersect earlier high-grade veins, mostly occurring as brecciated pyrite crystals with overprinted late Ag-tetrahedrite (up to 35 wt.% Ag) and pyrargyrite. The silver content of this stage suggests partial remobilisation of previously deposited metals, mostly due to re-brecciation followed by partial dissolution and Ag-sulphosalt precipitation.
The first three episodes are interpreted to have formed during the same event, probably from fluids of similar composition at a temperature of 290 to 230°C, dominated by mixed meteoric and volcanic waters (-3 to -0‰ δ18Owater), with a salinity of <3% NaClequiv., and a magmatic source of sulphur (-1 to -2‰ δ34Swater). Metals were predominantly precipitated during episode 1, at the beginning of vein formation. It was the result of a combination of boiling below the palaeowater-table at depths of ∼600 to 800 m, and was associated with mixing of meteoric and volcanic waters and the consequent cooling of the latter, as evidenced by sulphide-rich bands with crustiform-colloform quartz, adularia and chlorite-smectite banding.
Episodes 2 and 3 are characterised by a progressive decrease in the amount of metals being precipitated during boiling conditions, with veins being filled and their existing metal content being diluted by the addition of voluminous quartz and calcite. These episodes represented the waning stages of the hydrothermal system, and corresponded to the influx of bicarbonate waters (-6 to -8.5‰ δ18Owater).
Hydrothermal alteration is characterised by i). a proximal illite, adularia and a silica zone with chlorite and minor epidote, ii). intermediate interlayered illite-smectite and iii). a distal chlorite halo. This assemblage is consistent with measured fluid inclusion temperatures. The Marianas-San Marcos vein system is characterised by high-grade/high-temperature veins that are partially covered by felsic breccia and volcaniclastic deposits, and are spatially associated with overlying hot spring-related deposits and an advanced argillic alteration blanket. It has been proposed (Vidal et al., 2016) that during deposition, uplift and erosion brought deeper formed veins to the surface where they were partially in contact with and overlain by non-explosive, post-mineral rhyolitic domes and reworked volcaniclastic deposits, whilst shallow geothermal hot-spring deposits were telescoped onto them. As deeper veins were brought to shallower levels by uplift, boiling and mineralisation continued at depth, increasing the vertical extent of preserved mineralised veining. The last tectonic-hydrothermal episode 4, interpreted to have formed at lower temperatures, could be related to this late tectonic and hydrothermal activity.
Six adularia-rich vein samples from the Marianas-San Marcos vein system were analysed by 40Ar/39Ar geochronology (Vidal et al., 2016). All were from the main episode 1 mineralising event, representing a range of occurrence from fine to coarse grain-size. The results were found to be very consistent, with the average mean fusion age for all six samples being 154.92±0.72 Ma, taken as a preferred result for the timing of adularia crystallisation. Rocks above the Mariana Basin hot springs sequence are by inference and correlation with similar dated 154.6 to 147.6 Ma rocks from the San José deposit to the north, are interpreted to be younger than the age of mineralisation.
Reserves and Resources
Mineral Resources and Ore Reserves at 31 December 2015 (Tripp et al., for Goldcorp, 2015) were:
Measured resource - 1.35 Mt @ 4.99 g/t Au, 51.62 g/t Ag;
Indicated resource - 5.53 Mt @ 5.97 g/t Au, 38.58 g/t Ag;
Measured + Indicated resource - 6.88 Mt @ 5.78 g/t Au, 41.14 g/t Ag - for 39.8 t Au, 282.7 t Ag;
Inferred resource - 2.17 Mt @ 7.19 g/t Au, 44.68 g/t Ag - for 15.6 t Au, 96.7 t Ag.
Ore Reserves - not included in resources
Proved reserve - 5.02 Mt @ 10.58 g/t Au, 94.38 g/t Ag;
Probable reserve - 10.00 Mt @ 9.17 g/t Au, 64.81 g/t Ag;
Proved + Probable reserve - 15.02 Mt @ 9.64 g/t Au, 74.69 g/t Ag - for 145 t Au, 1122 t Ag;
TOTAL endowment = resources + reserves + production - 221 t Au, 1760 t Ag.
This record is largely drawn from: Tripp, A., Goodman, S., Pareja, G. and Murray, K., 2015 - Cerro Negro Operations, Santa Cruz Province, Argentina - an NI 43-101 Technical Report prepared for Goldcorp. Inc, 164p. and Vidal et al. (2016) as cited below.
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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Cerro Negro - Vein Zone
Vidal, C.P., Guido, D.M., Jovic, S.M., Bodnar, R.J., Moncada, D., Melgarejo, J.C., and Hames, W., 2016 - The Marianas-San Marcos vein system: characteristics of a shallow low sulfidation epithermal Au-Ag deposit in the Cerro Negro district, Deseado Massif, Patagonia, Argentina: in Mineralium Deposita v.51, pp. 725-748.|
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