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Onto

Sumbawa, Indonesia

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The Onto high sulphidation copper-gold epithermal/porphyry deposit is located near the south coast of the eastern portion of Sumbawa Island in West Nusa Tenggara Province, Indonesia, ~170 km east of Batu Hijau (#Location: 8° 50' 24"S, 118° 26' 39"E)

  Between 1995 and 1997, Aberfoyle Exploration, the manager at that time of Eastern Star Resources who held title of over the deposit area, conducted systematic exploration over the central parts of the project area. This program involved fixed wing aeromagnetic surveys, stream sediment and soil geochemical sampling. It did not locate the deposit, although some significant Mo soil geochemistry anomalies were located and a 9 × 5 km lithocap area of advanced argillic alteration was identified. This lithocap was characterised by quartz-alunite-dickite-kaolinite alteration with widespread silicification. Some 11 holes for a total of 5528 m of drilling tested geochemical anomalies (particularly soil Mo anomalies) and drilled under some of the silica ledges. The best results included a trench with 100 m @ 0.13% Cu and 0.23 g/t Au, and a best drill intersection of 106 m at 0.14% Cu and 0.08 g/t Au in andesite. Aberfoyle's share of Eastern Star Resources was sold to a private individual in 1999, who in 2009 negotiated an option agreement with, and in 2012 on-sold the company to the Vale Managed Sumbawa Timur Mining Joint Venture with an Indonesian partner. In 2011, Vale had drilled a coincident aeromagnetic high and soil Cu-Au-Mo-Pb-As anomaly at the Humpa Leu East prospect to intersect 414 m @ 0.45% Cu, 0.26 g/t Au, from 138 m. This prospect, which is ~2 km east of Onto, is associated with an early, pencil-shaped (150 x 80 m in plan) diorite intrusion with Cu-Au ±Mo in biotite-magnetite alteration and an A-B quartz veinlet stockwork. Subsequent drilling in 2011-12 outlined a small copper-gold resource with a grade of 0.5% Cu, 0.5 g/t Au.
  Encouraged, Vale continued exploring the lithocap by mapping, soil sampling and a high-resolution magnetic/radiometric survey. During 2012-13, deep drilling to ~800 m moved progressively to the southwest from Humpa Leu East. The deep holes were to intersect porphyry style mineralisation at depth below the lithocap. One of the final holes in the step-out program was to test the Onto prospect area, where a small anomalous magnetic high and coincident subtle Cu soil anomalies (six samples with 100 to 160 ppm Cu, but no other anomalous metals). Mapping had identified several covellite mineralised clasts within a probable late-mineral phreatic breccia, which was a key observation supporting the decision to drill the Onto prospect. Although not fully realised at the time, the subtle copper-only soil anomalies reflect the phreatic breccia dykes. This hole, drilled in June 2013, intersected from the surface down:
i). 390 m of relatively unaltered magnetite-bearing andesite flows, underlain by,
ii). ~100 m of quartz-dickite-kaolinite ±pyrophyllite alteration;
iii). an ~80 m thick, sub-horizontal silicified vuggy residual quartz horizon composed almost entirely of quartz with only minor alunite ±dickite;
iv). from 548 m down hole, it intersected 287 m @ 0.97% Cu, 1.13 g/t Au in vuggy-textured residual quartz, then quartz-alunite alteration. Copper is almost entirely present as covellite [CuS] with associated pyrite and native sulphur, and minor enargite [Cu3AsS4]. The hole ended in covellite-pyrite mineralisation with 0.6% Cu, 2 g/t Au in an intense quartz A-B vein stockwork within quartz-alunite and vuggy residual quartz alteration. It was stopped at 835 m, at the limit of the drills capacity.
  Additional drilling expanded the size of the potential deposit, with the fourth hole revealing the full vertical extent of the mineralisation. It was near vertical and was terminated at 1485 m, still in mineralisation, after intersecting 948 m @ 1.26% Cu and 0.7 g/t Au. Subsequent holes intersected similar intervals to the discovery hole, but several encountered higher-grade mineralisation in a interval at the top the main mineralised zone, over widths of 10 to 30 m, e.g., 24 m @ 6.9% Cu, 0.42 g/t Au. A substantial high grade indicated and inferred Mineral Resource had been outlined as of late 2020 (listed below). This resource is surrounded by a larger tonnage of 0.4 to 0.7% Cu and 0.2 to 0.3 g/t Au delineated by broader spaced drilling. The vast majority, i.e., >90% of these resources occur as covellite-pyrite mineralisation within quartz-alunite and quartz-pyrophyllite ±alunite and diaspore alteration zones.

Regional Setting

  Sumbawa Island is part of the Sunda-Banda volcano-magmatic arc and comprises an early Miocene to Holocene volcanic arc developed on oceanic crust of the Sunda Plate, above the NE directed subducting Australia-India oceanic plate (Garwin et al., 2005). The Batu Hijau and the Elang Project porphyry deposits are located in a similar setting near the south coast in the western third of Sumbawa, ~175 and 120 km to the west respectively (Garwin, 2002; Maryono et al., 2018), as is the Tujuh Bukit (Tumpangpitu) porphyry and high-sulphidation epithermal deposit on the south coast of eastern Java (Harrison et al., 2018). The stratovolcanoes that young to Quaternary in the northern half of Sumbawa, e.g., Tambora, indicates a northward migration of magmatic activity and a progressive temporal change from calc-alkaline in the older and more eroded southern margin of the island to shoshonitic affinities in the north (Garwin, 2002).
  U-Pb age dating of zircons (n=362) from heavy mineral concentrates collected from the six catchments draining the project area (GEMOC, 2013) indicate a progressive development of andesitic volcanism, with individual zircons recording magma chamber crystallisation age ranging from 5.2 to 0.3 Ma, with a principal age range of 2.7 to 0.3 Ma including peaks of 2.5 to 2.3, 1.7 to 1.5 and 1.1 to 0.7 Ma. It appears that even though the main focus of recent magmatic activity migrated northward, volcanism persisted in the south until at least 0.5 Ma. Seismo-tectonic activity (Pownall and Lister, 2015) indicates the Onto project area is near the intersection of several important fault zones. A major NW-trending sinistral fault is projected through the area, passing 15 km NE of Onto and continuing along the southwestern side of Tambora volcano. This structure is truncated by a major sinistral fault indicated along the southern coastline of Sumbawa, passing 5 km to the north of Onto. It also seems likely that a major NE-trending dextral fault runs up the bay directly to the west, passing ~20 km NW of Onto (Burrows et al., 2020).

Geology

  The volcano-sedimentary sequence and intrusive history of the deposit area is as follows (after Burrows et al., 2020):
Early Andesites - a volcano-sedimentary sequence that represent the oldest rocks in the Onto project area. They comprise a well-bedded sequence of mixed low-K calc-alkaline basalt to andesitic volcanic flows, volcanic breccia and sills with local sedimentary interflow horizons. The interflow sedimentary units include bright red, laminated cherts and hyaloclastic breccias, indicating parts of the sequence formed in subaqueous, perhaps submarine conditions. This sequence is found around the margins of the drilled deposit area, apparently representing the walls to the interpreted 1 x 1.5 km diatreme breccia described below.
Polymictic Diatreme Breccia - a complex, matrix to clast-supported polymictic breccia, typically unbedded and poorly to un-sorted over a vertical thickness of at least 1200 m. This is the main member of the volcano-sedimentary package at Onto. Clasts are predominantly rounded to subrounded porphyritic dacite and andesite, with lesser siliceous sedimentary rocks, vuggy residual quartz, quartz-alunite, pyritic sedimentary rocks and a range of porphyries, including a distinctive epidote-altered variety. Rare magnetite-altered and A-veined porphyry clasts have been encountered, as have porphyry fragments with elongate and amoeboid forms and cuspate margins resembling juvenile fragments. Localised drill intersections are characterised by largely monomictic clasts dominated by eutaxitic-like textures resembling flattened fiamme with a subhorizontal to 60° alignment. The polymictic breccia has been interpreted to have formed in a diatreme vent (e.g., White and Ross, 2011), the outer limits of which have not been fully defined as it does not appear to crop out. Drilling suggests it is at least 2 x 1.5 km in extent with an estimated vertical thickness of at least 1200 m. Matrix-supported polymictic breccia from this unit with weak illite smectite alteration has been dated at 0.716 ±0.081 Ma (LA_ICPMS U-Pb - zircon; Burrows et al., 2021).
Upper Sedimentary Package - which largely overlies and is gradational with the underlying polymictic diatreme breccia. The latter passes upwards into a 1 to 180, averaging 50 m thick bedded sedimentary sequence. It comprises finely laminated, locally carbonaceous, grey siltstones and tuffs, bedded polymictic breccias and volcaniclastic tuffs, as well as pyroclastic flows covering a NW-SE elongated area of >1.75 x 1 km. The bedded sedimentary rocks have graded and undulating bedding, abundant soft-sediment deformation, and commonly contain volcanic bombs and layers of accretionary lapilli. The volcaniclastic and pyroclastic units seem to have variable thicknesses, ranging from up to 75 m to being absent. The pyroclastic interval of this package has been dated at 0.772 ±0.087 Ma (LA_ICPMS U-Pb - zircon; Burrows et al., 2021). However, a sample from this same package with vuggy residual quartz alteration returned a date of 0.850±0.046 Ma (LA_ICPMS U-Pb - zircon; Burrows et al., 2021).
Capping Andesite - a relatively unaltered phenocryst-rich, low-K porphyritic andesite, which caps the Upper Sedimentary Package and forms most of the higher ground around Onto. It is generally 300 to 400 m thick in the central part of the deposit area, but is partially eroded to the NW. It has large, fresh plagioclase and hornblende phenocrysts and primary magnetite, but only rarely quartz phenocrysts. A few minor interbeds are recognised, but it is generally massive and relatively uniform without flow foliations or flow-aligned phenocrysts. It is interpreted to have formed in flow domes with talus breccias around the edges and a roughly planar lower contact. It very locally contains rounded xenoliths of magnetite-rich rock. Two samples from the Capping Andesite were dated at 0.709 ±0.063 Ma (LA_ICPMS U-Pb - zircon; Burrows et al., 2021) and 0.745 ±0.035 Ma (SHRIMP U-Pb - zircon; Burrows et al., 2021).
Phreatic Breccias, which are locally injected into the Capping Andesite and Upper Sedimentary Package as small steep, anastomosing, phreatic breccia dykes or pipes, characterised by rock-flour matrix, and sometimes occupying fault zones. Clasts are mainly from the surrounding andesite, although some are possibly sedimentary, as well as vuggy residual quartz fragments with rare covellite, suggesting a late origin from below the andesite flows. These breccias are generally weakly anomalous with 150 to 500 ppm Cu, variable 10 to 100 ppm As and 0.02 to 0.03 ppm Au.
Multiphase Porphyry Intrusions, occurring as a series of porphyry stocks cutting the Polymictic Diatreme Breccia to about 500 to 600 m depth below the current surface (~50 m below sea level [bsl]). These stocks coalesce downward to form a composite NW-trending 1.6 x 0.6 km intrusion at ~1000 m below the surface (~500 m bsl) expanding to 1.8 x 0.8 km at ~1500 m depth (~1000 m bsl). The recognition of intrusive phases within these porphyries is complicated by textural destruction resulting from pervasive advanced argillic alteration, with relict igneous porphyritic textures intensely altered to 40 to 60% quartz, 10 to 40% alunite, 5 to 20% pyrite ±pyrophyllite and diaspore. Never-the-less, three phases have been recognised, based on preserved features, including porphyritic textures, intensity of alteration, quartz flooding and intensity of A- and B-type quartz veinlets. Additional evidence of porphyry contacts includes xenoliths of early quartz veined porphyry clasts, or floating quartz veinlet xenoliths in the younger porphyry phases close to contact with older phases. Rarely, veinlets are truncated at contacts. The phases recognised are:
Early Porphyry - a series of small, <200 m wide vertical stocks that 'top out' at about 500 to 600 m below the surface but appear to expand and perhaps coalesce at depth as described above. It commonly has irregular, wavy, wormy-textured (gusano) A-type quartz veinlets typical of apical sections of porphyry stocks (e.g., Gustafson et al., 2004; Sillitoe, 2010). This early phase has the highest magnetite content, converted to hematite where subjected to advanced argillically alteration, and commonly >30% A and B quartz veinlet stockwork intensity. Some of the higher-grade mineralisation (≥1% Cu, ≥1 g/t Au and Cu/Au ~1:1) occurs within these stockworks at the top of the stocks, with quartz vein densities of up to >50 to 90% of the rock. Early Porphyry samples have been dated at 0.536 ±0.077 Ma (LA_ICPMS U-Pb - zircon; Burrows et al., 2021) and 0.688 ±0.053 Ma (
230Th corrected SHRIMP U-Pb - zircon; Burrows et al., 2021).
Early Inter-mineral Porphyry, occurring as stocks that cut the Early Porphyry. It is characterised by B veinlet densities of >15 per 5 m interval, the lack of intense fine magnetite alteration, and by A-B quartz veinlet stockworks. It typically carries grades of ~0.6 to 0.8% Cu, 0.2 to 0.4 g/t Au, and has a narrow range of Cu/Au ratios of ~3:1. Like the Early Porphyry it occurs as ~200 to 300 m diameter vertical stocks that also 'top out' some 500 to 600 m below the current surface, and commonly host gusano A-type quartz veinlets in their apical sections. The matrix of the porphyry is often strongly altered and quartz flooded, although plagioclase porphyritic textures are sometimes preserved. Quartz vein xenoliths are commonly found floating in, and partially assimilated by this intrusion within 2 to 4 m of contacts with the Early Porphyry.
Late Inter-mineral Porphyry - a plagioclase-hornblende phyric intrusion in which igneous textures can be well preserved and quartz flooding and obliteration of textures is rare. Xenoliths of rock and quartz veinlets from earlier porphyries are common along its contacts. Contacts with earlier intrusions and wall rock are often at low- to moderate angles suggesting intrusion as sills and dykes rather than as pencil shaped pipes like the two earlier porphyry phases. It has been intersected as shallow as up 500 m below the surface (~25 m bsl). B veins are sparse to rare, with densities of only 1 to 10 B vein per 5 m interval. Grades within this porphyry are typically ~0.4 to 0.6% Cu and 0.1 to 0.3 g/t Au, with higher Cu grades related to late veins of pyrite-covellite. It has a characteristic Cu/Au ratio of ~3:1 (%:g/t). This porphyry was dated at from 0.4 to 1.4 Ma, with a weighted, Th-corrected average of 0.657 ±0.075 Ma.(
206Pb/238U; Burrows et al., 2021).
Late to Post-mineral Intrusions - two later intrusions are differentiated, comprising i). plagioclase ±hornblende andesite dykes and ii). flat-lying sills of quartz-phyric andesitic to dacitic composition, which less commonly occur as dykes, usually within or close to the base of the Upper Sedimentary Package or high in the Polymictic Diatreme Breccia Unit. The andesite dykes are considered to possibly be feeders to the overlying Capping Andesite flow domes, whilst the sills are consistently present and are known over a strike length of at least 1 km. Both intrusion are affected by the advanced argillic alteration making recognition difficult, although quartz eyes in the sills are preserved through even the most intense advanced argillic alteration. Both intrusions lack A or B veins, but do contain high sulphidation pyrite. The dykes in particular are often less altered to smectite or argillic assemblages in contrast to the surrounding rocks, indicating they may postdate some of the advanced argillic alteration event.

Alteration

  Alteration in the immediate Onto deposit area principally comprises the remnants of a >30 km
2 lithocap. Only a few drill holes have passed through the base or margins of this advanced argillic alteration package and intersected the underlying porphyry-related alteration. The two alteration types are described separately below.

Lithocap Alteration
  Drill hole logging has defined a very thick zone of intense advanced argillic alteration covering an area of at least 1.5 x 1.0 km in the immediate deposit area with a thickness of >1200 m in its central portion. Several of the deeper holes have bottomed in advanced argillic alteration at 1500 m below their collar. This alteration is well developed in the polymictic breccia but also occurs within all the intrusion types and also, to a limited extent and less intensely in the early andesites. The lithocap thins peripheral to the central part of the main deposit where holes ~1 km out from core of system still pass through between 400 and 500 m of lithocap. The principal alteration styles of the lithocap are as follows (after Burrows et al., 2020):
Illite-Smectite to Unaltered - the bulk of the Capping Andesite flows are essentially unaltered, with the primary Na
2O contents of ≥2% preserved, along with Ba of >400 ppm, relatively high Sr of >250 ppm and V of >100 ppm, whilst sulphur values are also generally ≤4%. However, in the lower sections, the andesite flows are increasingly altered, with fracture controlled illite-smectite becoming increasingly pervasive to form a light green-coloured smectite, identified as montmorillonite, with illite. In some peripheral drill holes, illite-smectite in fractures overprints chlorite and chlorite-epidote alteration, possibly representing pre-existing regional propylitic alteration. In some areas, Illite-smectite alteration zones are consistently found below the quartz-dickite zone. However, the uppermost illite-smectite alteration is interpreted to have occurred at <200°C.
Quartz-Dickite with kaolinite ±pyrophyllite define a discontinuous 100 m thick zone capping the advanced argillic alteration, interspersed with smectite dominated alteration zones. This is interpreted to indicate the responsible fluid had cooled and became less acidic upward into the Capping Andesite. On the edges of the system, the quartz-dickite assemblage very occasionally overprints illite-smectite alteration in sub-horizontal fractures before more pervasively altering the rock. Quartz-dickite is interpreted to have occurred at ~200 to <270°C.
Porous to Friable Vuggy Residual Quartz forms a subhorizontal layer which typically occurs within or toward the base of the quartz-dickite alteration and commonly contains abundant dark, fine-grained pyrite. Vuggy quartz textures are well developed in the Capping Andesites, but are often more porous and friable in the Upper Sedimentary Package or in the top of the Polymictic Breccia. The horizon roughly corresponds to the base of the capping andesite, as well as the present water table at 180 to 200 m asl. To the northwest, where it is exposed by erosion, a bleached spongy white silica-kaolinite-sulphur ±alunite alteration appears to have overprinted portion of the outcropping earlier quartz-dickite-kaolinite and vuggy residual quartz alteration.
Silicic Alteration which forms a gently dipping, subhorizontal layer of vuggy residual quartz and silicification that everywhere marks the top of the much more pervasive and intense quartz-alunite to quartz-pyrophyllite alteration zone. The majority of this horizon is a massive, completely silicified rock, although occasionally vuggy textures remain, indicating this episode of silicification overprinted an earlier vuggy residual quartz horizon. It is consistently developed as a 100 to 200 m thickness, occasionally to 300 m and occurs at about sea level, i.e., ~450 to 500 m below the surface, and has very sharp upper and lower contacts. It is characterised by <1% Al
2O3, commonly <0.5%, and intense leaching of most elements, including Ti and Zr, indicative of low pH fluids that dissolve everything but quartz and pyrite. Hydrothermal monazite and zircon are recognised. Vuggy quartz zones are also commonly developed within quartz-alunite alteration lower in the sequence, but do not there have the same extreme Al depletion, usually still carrying >2 to 9% Al2O3.
  The silicic horizon is interpreted to represent ponding of cooling vapour condensate below ≤200°C at low pH of <1, where only quartz, pyrite and native sulphur are stable, and alunite dissolves (Hedenquist and Taran, 2013), forming vuggy residual quartz, which once formed was overprinted by pervasive silicification. The persistent nature, sharp contacts and subhorizontal attitude of this layer at Onto is taken to suggest formation beneath a paleowater table, as silica saturated water cooled and precipitated quartz proximal to that water table (Longo, 2000).
Quartz-Alunite alteration predominates below the silicic horizon with subordinate pyrophyllite, although thin intervals are also locally found above the silicic horizon. It is a light-coloured alteration, and has a patchy texture where dickite, alunite and pyrophyllite preferentially replace clasts in the Polymictic Diatreme Breccia protolith. Locally, up to 5 vol.% lilac-blue dumortierite occurs sporadically throughout the quartz-alunite zone, as well as in the quartz-pyrophyllite ±diaspore zone described below. Within the Polymictic Diatreme Breccia, dumortierite is usually found within a few hundred metres of porphyry intrusions. In the greater part of the Onto deposit area, the older, higher temperature quartz-pyrophyllite ±diaspore alteration is partially to completely overprinted by thick quartz-alunite alteration zones, typically dominated by K alunite, especially in the upper parts. This alteration locally extends as consistent roots down through the quartz-pyrophyllite-diaspore in steep to vertical zones. It is generally more porous with a greater proportion of vuggy-textured residual quartz developed in subhorizontal zones and less commonly as the steep to vertical zones. Modeling indicates alunite is stable between ~200 and 300°C and dominates in abundance over aluminosilicate minerals under more oxidised conditions, and that pyrophyllite becomes increasingly unstable under higher water/rock ratios (Hedenquist and Taran, 2013).
Quartz-alunite alteration at Onto is characterised by Al
2O3 concentrations of from <10 to ~6.5 wt.%, although lower values reflect localised vuggy-textured residual quartz development in areas of more intense leaching and/or quartz flooding.
  Sodic alunite from quartz-alunite altered Polymictic Diatreme Breccia from relatively shallow levels, above the silicic horizon, returned some of the oldest
40Ar/39Ar ages of 0.98 ±0.22 and 0.72 ±0.19 Ma, whilst samples from deeper in the system, closer to the base of the quartz-alunite zone, were dated in the range 0.420 ±0.210 to 0.251 ±0.039 Ma. Late alunite crystals from vugs lined with alunite and covellite were dated at 0.056 ±0.021 and 0.038 ±0.031 Ma (Burrows et al., 2020).
  Numerous quartz-alunite and vuggy residual quartz as well as mineralised veined-porphyry clasts occur in the Diatreme Polymictic Breccia, which pre-dates the Early Porphyry. This suggest an early phase of porphyry mineralisation and lithocap development had occurred prior to diatreme formation, although the exact timing and extent of this event has not been established (Burrows et al., 2020). This may also suggest any such earlier mineralisation is related to a deeper porphyry event, or has been obliterated by the breccia and Early to Late Porphyry intrusions.
Quartz-Pyrophyllite ±Diaspore is the most widespread, deepest, earliest and highest temperature advanced argillic alteration zone, generally occurring at ≥900 m below surface (≥400 m bsl). It is accompanied by variable alunite, topaz, zunyite, dumortierite, Na-alunite and pyrite, as well as corundum and andalusite. It is very distinctive with a compact brownish to tan-coloured matrix of pyrophyllite and/or diaspore, surrounding ragged, whitish patchy to ovoid textures representing resorbed and modified clasts of the protolith breccia. The top section is dominantly composed of quartz-pyrophyllite, whilst diaspore with topaz and zunyite gradually become more abundant downward, generally below ~600 m bsl. The overprinting boundary of this earlier high-temperature alteration with later quartz-alunite is often transitional rather than sharp and often occurs as a hybrid of the two types. Upper quartz-pyrophyllite ±diaspore patches in the transition are preserved from 470 to 75 m bsl, occasionally persisting to as shallow as sea level. It has been experimentally demonstrated that pyrophyllite and diaspore should not coexist at <280° (Hemley et al., 1980), whilst zunyite is stable up to 450°C, to be replaced by topaz at higher temperatures (Hsu, 1986). Burrows et al. (2020) therefore conclude that the quartz-pyrophyllite-diaspore alteration formed at temperatures of ≥300°C from ascending magmatic vapour, following brine-vapour separation in the porphyries below.
 
40Ar/39Ar age dating of alunite associated with topaz and diaspore from this alteration zone returned ages between 0.8 and 0.3 Ma (Burrows et al., 2020). The oldest reliable age in this range overlaps the U-Pb zircon age from the Early Porphyry. This suggests advanced argillic alteration was taking place (or continued to take place from an earlier pulse) during or immediately following emplacement of the porphyry stocks, potentially over an at least 1 km of vertical interval (Burrows et al., 2020).

The advanced argillic alteration zonation is intact at Onto, and in the immediate deposit area there is little indication at surface of the quartz-alunite-pyrophyllite alteration that hosts mineralisation. The exception is to the NW, in the Wadubura area, where the lithocap partially crops out and quartz-dickite and vuggy quartz alteration is overprinted by a porous white, powdery, steam-heated assemblage composed of cristobalite, kaolinite and sulphur, ±sodic alunite. This surface alteration is continuous with, but probably overprints, the upper porous, vuggy quartz horizon. As such it may represent the palaeo-shallowest expression of the advanced argillic alteration.

Porphyry-Style Alteration
  The following styles of alteration have been encountered in the porphyry regime below the lithocap:
Potassic alteration is occasionally differentiated by the presence of hydrothermal magnetite and biotite ±K feldspar, commonly weakly overprinted by quartz-alunite, quartz-dickite and/or quartz-kaolinite. Where present, it is also usually overprinted to varying degrees by chloritic alteration. Blocks of relict potassic or chloritic alteration are sometimes found within quartz-alunite and quartz-pyrophyllite-diaspore alunite alteration, surrounded by hematite (martite) after disseminated or veinlet magnetite. Potassic alteration has only been seen in the very bottom of a few drill holes and between 738 and 1090 m depth marginal to the advanced argillic-altered package in another. A- and B-type quartz veinlets, which accompany high-temperature potassic alteration in many porphyry copper deposits, are widespread within the deep lithocap at Onto over intervals of 300 to 400 m.
Phyllic-Chloritic alteration, characterised by a muscovite/sericite-quartz ±chlorite assemblage, commonly with pyrite, has been found at the bottom of several deep drill holes, where it appears to overprint the potassic assemblage, as indicated by remnant magnetite. It is rarely very extensive, and is commonly overprinted by weak argillic or quartz-dickite assemblages. It more commonly occurs in the Early Porphyry and Early Inter-mineral Porphyry and rarely in the Late Intermineral Porphyry and Polymictic Breccia wall rock.
  Limited (n=2) Re-Os dating of molybdenite from B vein porphyry mineralisation in the phyllitic alteration zone returned dates of between 0.44 and 0.35 Ma. Similarly limited dating of molybdenite from high-sulphidation style banded pyrite-alunite-quartz molybdenite veins yielded ages of 0.36 ±0.27 and 0.41 ±0.04 Ma, i.e., in the same age range as that in the porphyry style mineralisation.
Propylitic alteration, signified by an assemblage of chlorite-epidote-calcite ±actinolite, observed at the base of several drill holes, usually as relict alteration in less permeable lithologies, and was originally thought to be propylitic alteration related to the porphyry intrusions. However, drill holes to the north of Onto, beyond the advanced argillic alteration halo, typically pass through illite-smectite to chlorite to chlorite-epidote alteration at depth, suggesting chlorite-epidote alteration was a product of regional alteration from near-neutral pH fluids un-related to the porphyry system.

Mineralisation

  The shallowest mineralisation is a distinct zone of Ag-As-Sb-Bi ± minor Cu and Au enrichment, accompanied by fine-grained dark pyrite. This zone occurs toward the base of the silicic alteration horizon, associated with fracturing developed in that interval. In the SE of the deposit, this zone directly overlies the main covellite-pyrite dominated copper-gold mineralisation that appears in the quartz-alunite alteration directly below the silicic zone at approximately sea level, which is between 300 and 600 m below surface. To the NW, at Wadubura, copper-gold mineralisation within the 0.3% Cu grade shell, is ~150 m below the basal contact of the shallowly SE dipping silicic horizon. Many drill holes were terminated in mineralisation at 1200 to 1500 m downhole, corresponding to ~800 m bsl.
  Two main mineralisation styles are evident: i). intense A-B quartz veining related to porphyry stocks which appear to represent a porphyry environment and ii). sulphides with high-sulphidation states that are more typical of a epithermal lithocap environment. However, unlike many other linked porphyry high-sulphidation epithermal systems, there appears to be a complete spatial, and possibly temporal telescoping/overlap of the two styles, as follows.
Chalcopyrite-bornite related to biotite-magnetite alteration - which has been encountered on the margins, and rarely, below the advanced argillic alteration, within the Early, Early Inter-mineral and to a lesser extent the Late Inter-mineral Porphyries. This association is represented by low- to intermediate-sulphidation assemblages of chalcopyrite ±pyrite ±bornite ±digenite/chalcocite associated with magnetite and hydrothermal biotite alteration, plus A- and B-type quartz veinlet stockworks. Grades are variable, but generally ~1% Cu and ~1 g/t Au, with gold grades of >1 g/t typically associated with intervals of bornite. The chalcopyrite-bornite association has very low relative As values, e.g., one drill hole intersecting intervals of porphyry mineralisation with little advanced argillic overprint averaged only 8 ppm As, 3% S, whilst another intersection overprinted by advanced argillic alteration averaged 0.93% Cu, 1.34 g/t Au, 44 ppm As, 5.3% S. In the same hole, the Early Intermineral Porphyry with potassic alteration grades ~0.53% Cu, 0.23 g/t Au, 7 ppm As, 3.8% S. Overall, porphyry-style chalcopyrite-bornite ±pyrite mineralisation only constitutes a small proportion (~8%) of the resource at Onto as of late 2020.
High-sulphidation Mineralisation - apart from the small quantity of porphyry-style detailed above, mineralisation at Onto is characterised by a high-sulphidation assemblage of covellite-pyrite ±native sulphur, very minor enargite and trace amounts of digenite and chalcocite. This assemblage is associated with quartz-alunite-pyrophyllite dominated alteration. Nukundamite [(Cu,Fe)
4S4] has also been recognised, intergrown with both pyrite and covellite, emphasising the high sulphidation character this mineralisation. Burrows et al. (2020), after Inan and Einaudi (2002), note that nukundamite is stable at temperatures between 501 and 223°C at very high sulphidation states and higher values of ƒS2 and ƒO2 than chalcopyrite+bornite and chalcopyrite+pyrite, assemblages. Barite is also common in the upper portions of the mineralised zone.
  In excess of 90% of the Mineral Resource delineated by late 2020 (as listed below) lies within the quartz-pyrophyllite-alunite and quartz-alunite alteration zones. Mineralisation of this type within the 0.3% Cu cutoff envelope has a bulk mineralogy of ~50 to 60% quartz, 5 to 15% alunite and 10 to 25% pyrite, with up to 5% pyrophyllite, dickite and kaolinite, a few percent covellite, 11% S and 9% Fe.
  Petrographic studies show that covellite and pyrite are typically late relative to quartz and alunite. Covellite often grows into open space. However, it is also commonly intergrown with, and/or is overgrown by, quartz, alunite and native sulphur. Covellite is also intergrown with a late, spongy, growth-banded pyrite (pyrite II) that overgrows cores of inclusion-free pyrite I. Rarely, quartz-alunite halos have been discerned fringing pyrite-covellite veins, suggesting these veins formed coeval to quartz-alunite alteration. Preliminary geometallurgical studies indicate two-thirds of the gold occurs as small inclusions in pyrite > covellite, with the remaining third as lattice gold in pyrite >> covellite. In contrast, silver occurs mainly as matildite (AgBiS
2), with ~25% contained in lattice sites in enargite and pyrite.
  The high sulphidation mineralisation occurs in the following forms:
Disseminated fine- to locally coarse grained covellite-pyrite within the porphyry intrusions and polymictic breccia. The Early Porphyry intrusion, in particular, hosts covellite occurring as coarse (several mm) crystals and rosettes with only minor pyrite. This mineralisation replaces the original wall rock between veins, and to a lesser extent, covellite is disseminated within the quartz veinlets. Covellite is intergrown with pyrophyllite, alunite, dickite, pyrite, native sulphur and, more rarely, anhydrite. All preexisting copper sulphides are completely replaced on a fine scale by covellite-pyrite. Chalcopyrite and bornite ±chalcocite are only preserved internal to A-B quartz veinlets as rare micro-inclusions within pyrite, in covellite, or enclosed within quartz.
Vug infilling of covellite-pyrite-native sulphur in quartz-alunite alteration zones. This assemblage is precipitated in vuggy quartz textures which are developed by the leaching of earlier minerals during alteration. Pyrophyllite, then alunite, become unstable in late fluids and are progressively leached, producing vugs that lead to a highly permeable rock that is sequentially infilled with pyrite, covellite and native sulphur. Pyrite-covellite veins with quartz-alunite alteration halos occur locally, indicating mineralisation also formed early, before alunite became unstable, in association with quartz-alunite alteration. Covellite is characteristically coarse and crystalline, commonly intergrown with pyrite but also with native sulphur and alunite, and more rarely with dickite in open space fillings.
Discrete veins and veinlets of pyrite-covellite that are typically 5 to 10 and locally up to 50 cm thick, and dip steeply, particularly near the top of the ore zone, just beneath the silicic horizon. A sub-type occurs within and adjacent to faults that cut the silicic horizon, with covellite dominated veins and replacement zones within the silicic horizon. Gold grades are typically low, with Cu/Au (%:g/t) ratios ≥5:1.

Synthesis

  The principal copper and gold mineralising event is interpreted to have taken place during emplacement of the Early, Early Inter-mineral and to a lesser extent the Late Inter-mineral Porphyry intrusive phases which cover a lateral area of ~1.6 x 0.6 km at 500 m bsl (below sea level). These porphyries were intruded at just after 0.688 ±0.053 Ma into the core of a diatreme breccia that has a surface area of at least 1.5 x 1 km and a vertical extent of >1.2 km. This steeply plunging diatreme was intruded through a shallowly dipping, early andesitic volcano-sedimentary sequence. The diatreme vent breccia had been capped by maar sediments and then andesite flows at just after 0.838 ±0.039 Ma, prior to the porphyry intrusion (Burrows et al., 2020). However, the presence of clasts of mineralised veined-porphyry and advanced argillic alteration within the Diatreme Polymictic Breccia, suggest there may have been a related earlier syn-mineral porphyry that is either at a greater depth, or has been obliterated by the diatreme and subsequent intrusions.
  The preserved porphyry style mineralisation of the main event was developed in the upper sections of these intrusions, and in the surrounding Polymictic Diatreme Breccia. It occurs over a ~350 m vertical interval, mainly between 100 and 500 m bsl, and is ~60% hosted by the porphyry intrusions, with the balance within the Diatreme Polymictic Breccia, largely above the stocks. This mineralisation, was subsequently overprinted by an intense advanced argillic lithocap, with only relicts remaining to indicate the presence and extent of porphyry mineralisation. These preserved relicts are found peripheral to, and in some cases, beneath the advanced argillic alteration package and account for <8% of the resource, as known in 2020. Where preserved, the relict porphyry-style mineralisation comprises disseminations and intense, high-temperature, A-, B, and A-B quartz veinlet stockwork shells of chalcopyrite-bornite-pyrite with associated potassic (secondary biotite-magnetite±K feldspar) and chlorite-sericite alteration. Rarely, 5 to 50 µm inclusions of chalcopyrite-bornite, and even more rarely bornite-chalcocite, are encapsulated within pyrite associated with covellite of the advanced argillic/high sulphidation assemblage. These sulphides are found adjacent to or within early A-quartz veinlets, reflecting remnants of the porphyry-style sulphide assemblage (Burrows et al., 2020).
  From the porphyry-style relicts, it is estimated the pre-overprint mineralisation averaged 0.8% Cu, 0.76 g/t Au in the Early Porphyry; 0.58% Cu, 0.25 g/t Au in the Early Inter-mineral Porphyry; and 0.41% Cu, 0.13 g/t Au in the Late Inter-mineral Porphyry, with As levels in all three of ~120 ppm. In the most pristine material As values are <45 ppm, with ~4.7% S (Burrows et al., 2020).
  The intense advanced argillic alteration referred to above, overprints the porphyry intrusions, diatreme, and to a lesser degree, the surrounding early andesite sequence walls of the diatreme. It is predominantly composed of quartz-alunite and quartz-pyrophyllite ±diaspore assemblages, and a dominant high-sulphidation style covellite-pyrite mineralisation (Burrows et al., 2020). This overprint was initiatedwhen the overpressured regime of the porphyry system had been breached. This overpressure was the result of lithostatic plus hydrothermal fluid pressures in a sealed/capped system that hydraulically opened and re-opened fractures to be filled by stockwork veining and precipitated sulphide assemblages as temperatures gradually subsided, or when partial pressure escape episodes occurred. Prograde potassic porphyry alteration occurs over the range of 600 to 450°C. When a major breach in the overpressured, largely sealed system occurred, the pressure declined relatively rapidly towards hydrostatic, the porphyry fracture-vein system collapsed and its sulphide deposition ceased. The reduced pressure resulted in brine-vapour separation, with the breach forming an escape corridor for ascending magmatic vapour from the deep parental magma chamber. This vapour was more acidic, and began the retrograde advanced argillic alteration and associated precipitation of high sulphidation mineral assemblages under declining temperatures.
  This progressive retrograde alteration commenced with the high temperature quartz-pyrophyllite ±diaspore assemblage generated at temperatures of between 450 and 300°C, with associated alunite dated at between 0.8 and 0.3 Ma, commencing very soon after the very earliest porphyry emplacement. As the vapour and condensed fluids cooled to below 300°C, pyrophyllite became unstable and alteration progressed to the quartz-alunite phase. Alunite from the quartz-alunite stage has been dated at between 0.420 ±0.210 to 0.251 ±0.039 Ma. As the vapour/fluids cooled, isotherms progressively retreated deeper into the lithocap, probably also influenced by meteoric waters, progressively replacing the quartz-pyrophyllite ±diaspore zone by quartz-alunite. At a higher level, interpreted to generally correspond to the water table and/or the overlying less permeable Upper Sedimentary Package, cooling and condensing vapour was ponded. As the temperature declined to ≤200°C at a low pH of <1, only quartz, pyrite and native sulphur were stable, and a 100 to 300 m thick silicic layer was developed. This impermeable layer, only hosts minimal mineralisation unless heavily fractured. It effectively capped and terminated further upward fluid flow that was still emanating from the deep parental magma chamber. Instead, the bulk of fluid flow was focused laterally to form the widespread quartz-alunite lithocap below the silicic zone. Above the silicic zone, the preceding fluid or that which escaped from or circumvented the silicic barrier was cooler and less acidic on the edges of the system and upward into the Capping Andesite. Quartz-dickite alteration became more stable in these conditions at ~200 to <270°C, largely occurring above or lateral to the silicic zone. The uppermost illite-smectite alteration layer is interpreted to have developed at <200°C and lower acidity, closer to the surface, but is locally overprinted by pulses of quartz-dickite along local aquifers (Burrows et al., 2020).
  The intense high-sulphidation overprint was associated with the further introduction of ~300 ppm As, an average of 11% S and other metals, including Cu and Au. Despite this, the mineralised volume, as defined by the 0.3% Cu cutoff shell, spatially migrated little from that of the initial porphyry system with 60% of the resource within the porphyry stocks. This is emphasised by peripheral drill holes that cut equally intense quartz-alunite-pyrophyllite alteration on the margin of the deposit, but do not contain significant Cu-Au mineralisation compared to those in the central porphyry intrusion cluster. Compared to the grades quoted above for porphyry-style mineralisation alone, grades within 0.3% Cu envelope, and predominantly in quartz-alunite-pyrophyllite ±diaspore alteration, average ~1.2% Cu, 0.7 g/t Au in the Early Porphyry; 0.9% Cu, 0.4 g/t Au in the Early Inter-mineral Porphyry; and 0.6% Cu, 0.3 g/t Au in the Late Inter-mineral Porphyry. In addition, the distribution of grade and copper-gold ratios, at least in the two earliest porphyry phases, are quite similar to those preserved in potassically altered porphyry-style mineralisation, indicating the quartz-alunite-pyrophyllite alteration and the high-sulphidation overprint do not radically change copper and gold distribution (Burrows et al., 2020).
  Following the breach of the overpressured porphyry system and subsequent cooling, Burrows et al. (2020) postulate metal-bearing fluids rapidly evolved to a very high sulphidation state due to the lack of a rock buffer in the thick, advanced argillic alteration (e.g., Einaudi et al., 2003). Burrows et al. (2020) also conclude there was no significant time gap between the 'ground preparation' through dissolution of exiting minerals by the highly acidic fluids, and the deposition of sulphides in the open space thus generated. This resulted in deposition of covellite-pyrite and widespread native sulphur, with covellite being intergrown with pyrophyllite, alunite, dickite and native sulphur, occurring as coarse-bladed crystals in pyrophyllite and in diaspore aggregates. Along with pyrite it also occurs as fine inclusions in zunyite. These relationships suggest covellite deposition spanned a wide temperature range, possibly from ≥300 to ~100°C (e.g., Reyes, 1990; Watanabe and Hedenquist, 2001). The original high-temperature porphyry style copper sulphide assemblages within and adjacent to A-B quartz veinlets were totally replaced by covellite during this process. Subsequently, further covellite, accompanied by pyrite, filled the open space created by the advanced argillic alteration, as described above. Some of the higher-grades are hosted by vuggy residual quartz within the quartz-alunite alteration and interstitial to relict quartz veinlet stockworks, where all but the quartz vein material was leached away. In addition to being intergrown with high-temperature minerals, covellite and pyrite occur as delicate intergrowths in open space filling with alunite, dickite and native sulphur, whilst pyrite-covellite veins sometimes are enclosed by alunite alteration halos. These observations are taken to indicate covellite-pyrite precipitation in both settings is closely related to on-going advanced argillic alteration by low-temperature, low-pH fluids, rather by temporally separated fluids that created and filled the porosity respectively (Burrows et al., 2020).
  Burrows et al. (2020) note that the porphyry- and high-sulphidation styles of mineralisation overlap very closely spatially and, based on Re-Os dating were temporally close at ~0.4 Ma. The maximum elevation difference between the crest of the now eroded overlying Onto volcanic edifice and the current land surface is estimated to be <600 m. Removal of this thickness has taken <0.4 m.y., implying relatively low degradation rates, rather than a large mass-wasting event normally expected to produce telescoping (Sillitoe, 1994). As such, the zoned advanced argillic alteration appears to have only ever extended ~250 to 300 m above the apices of the porphyry stocks and only reached the surface very locally in the NW section of the deposit area, although alteration extends to vertical depths that locally exceed 1200 m (Burrows et al., 2020). This implies the porphyry mineralisation was emplaced at abnormally shallow levels, and once its overpressure regime was breached, high sulphidation epithermal alteration, restricted above by the overlying Upper Sedimentary Package, was at already at sufficiently shallow depths to precipitate in situ, catalysed by the sulphides and pH-Eh conditions of the existing porphyry mineralisation. However, if the apparently older mineralised and altered clasts in the Polymictic Diatreme Breccia, represent an earlier syn-mineral intrusion and coeval mineralisation, in the absence of the overprinting diatreme, the cumulative porphyry column may have extended to much greater relative depths below the lithocap.
  The Onto mineralisation appears to have formed rapidly in the middle Pleistocene, between ~0.7 Ma and the present, with copper-gold mineralisation likely spanning ~100 k.y., between 0.44 and 0.35 Ma (Re-Os model Molybdenite ages), followed by remobilisation and additional of Cu and Au to as recently as ~38 k.y. (Burrows et al., 2020), from the same deep parental magma chamber that was the source of hydrothermal fluids and intrusions involved in the porphyry mineralisation.

Resources

According to Burrows et al. (2020) a preliminary resource estimation comprises:
    Indicated Mineral Resource - 0.76 Gt @ 0.93% Cu, 0.56 g/t Au, 5 g/t Ag, 350 ppm As, for 7 Mt of contained Cu, ~400 t Au;
    Inferred Mineral Resource - 0.96 Gt @ 0.87% Cu, 0.44 g/t Au, 3 g/t Ag and 350 ppm As for 8.3 Mt of contained Cu, ~435 t Au.

In addition, drilling on a 200 x 200 to 400 x 400 m pattern, insufficient to estimate a Mineral Resource, have suggested an exploration target of a further 0.6 to 1.7 Gt at 0.4 to 0.7% Cu, 0.2 to 0.3 g/t Au (Burrows et al., 2020 after P.T. Sumbawa Timur Mining, 2020).

This summary is based on, and closely follows, Burrows et al., 2020. The Synthesis above is based on the same source, but with some embellishments and alternate interpretations by this author.

The most recent source geological information used to prepare this summary was dated: 2020.    
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.


  References & Additional Information
   Selected References:
Burrows, D.R., Rennison, M., Burt, D. and Davies, R.,  2020 - The Onto Cu-Au Discovery, Eastern Sumbawa, Indonesia: A Large, Middle Pleistocene Lithocap-Hosted High-Sulfidation Covellite-Pyrite Porphyry Deposit: in    Econ. Geol.   v.115, pp. 1385-1412.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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