Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence
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Alunite
Argillic alteration
Hypogene
Stockwork
Phenocryst
Abstract Ninety-eight underground diamond holes (~102 km) drilled by Far Southeast Gold Resources Inc. at the Far Southeast porphyry Cu-Au deposit, Philippines, from 2011 to mid-2013, provide a three-dimensional exposure of the deposit between 700- and –750-m elevation, with surface at ~1,400-m elevation. Far Southeast contains an inferred resource of 891.7 million tonnes (Mt) averaging 0.7 g/t Au and 0.5 wt % Cu, equivalent to 19.8 Moz Au and 4.5 Mt Cu. This contribution reports the spatial and temporal distribution of alteration and mineralization at Far Southeast, notably a white-mica–chlorite-albite assemblage that formed after early secondary biotite and before late quartz–white-mica–pyrite alteration and that is associated with the highest copper and gold grades. Alteration assemblages were determined by drill core logging, short-wavelength infrared (SWIR) spectral analysis, petrographic examination, and a quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) study. Alteration is limited around sparse veins or pervasive where vein density is high and the alteration halos coalesce. The alteration and mineralization zones with increasing depth are as follows: (1) the lithocap of quartz-alunite–dominated advanced argillic-silicic alteration that hosts part of the Lepanto high-sulfidation Cu-Au epithermal deposit (mostly above ~700-m elevation), (2) an aluminosilicate-dominated zone with coexisting pyrophyllite-diaspore ± kandite ± alunite and white mica (~700- to ~100-m elevation), (3) porphyry-style assemblages characterized by stockwork veins (below ~500-m elevation), (4) the 1 wt % Cu equivalent ore shell (~400- to –300-m elevation), and (5) an underlying subeconomic zone (about –300- to –750-m elevation, the base of drilling). The ore shells have a typical bell shape centered on a dioritic intrusive complex. The paragenetic sequence of the porphyry deposit includes stage 1 granular gray to white quartz-rich (± anhydrite ± magnetite ± biotite) veins with biotite-magnetite alteration. These were cut by stage 2 lavender-colored euhedral quartz-rich (± anhydrite ± sulfides) veins, with halos of greenish white-mica–chlorite-albite alteration. The white mica is largely illite, with an average 2,203-nm Al-OH wavelength position. The albite may reflect the mafic nature of the diorite magmatism. The quartz veins of this stage are associated with the bulk of copper deposited as chalcopyrite and bornite, as well as gold. Thin Cu sulfide (chalcopyrite, minor bornite) veins with minor quartz and/or anhydrite (paint veins), with or without a white-mica halo, also occur. These veins were followed by stage 3 anhydrite-rich pyrite-quartz veins with white-mica (avg 2,197 nm, illite)–pyrite alteration halos. Combined with previous studies, we conclude that this porphyry system, including the Far Southeast porphyry and Lepanto high-sulfidation Cu-Au deposits, evolved over a period of 0.1–0.2 m.y. Three diorite porphyry stocks were emplaced, and by ~1.4 Ma biotite-magnetite–style alteration formed with quartz-anhydrite veins and deposition of ≤0.5% Cu and ≤0.5 g/t Au (stage 1); coupled with this alteration style, a barren lithocap of residual quartz with quartz-alunite halo plus kandite ± pyrophyllite and/or diaspore formed at shallower depth (>700-m elevation). Subsequently, lavender quartz and anhydrite veins with bornite and chalcopyrite (high-grade stage, avg ~1 wt % Cu and ~1 g/t Au) and white-mica–chlorite-albite halos formed below ~400-m elevation (stage 2). They were accompanied by local pyrite replacement, the formation of hydrothermal breccias and Cu sulfide (paint) veins. Stage 2 was followed at ~1.3 Ma by the formation of igneous breccias largely along the margins of the high-grade zones and stage 3 pyrite-quartz-anhydrite ± chalcopyrite veins with white-mica (mostly illitic) halos. At shallower depths in the transition to the base of the lithocap, cooling led to the formation of aluminosilicate minerals (mainly pyrophyllite ± diaspore ± dickite) with anhydrite plus high-sulfidation-state sulfides and pyrite veinlets. Consistent with previous studies, it is likely that the lithocap-hosted enargite-Au mineralization formed during this later period.
Alunite
Sericite
Stockwork
Hypogene
Argillic alteration
Cassiterite
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Abstract The Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The ore deposits are hosted by Middle Jurassic andesitic to dacitic volcanic and volcaniclastic rocks of tholeiitic to transitional affinities below a late Oxfordian unconformity, which is covered by calc-alkaline to transitional Late Jurassic-Early Cretaceous volcanic rocks interlayered with sedimentary rocks. The mineralization consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulfidation mineral assemblages with enargite and luzonite. Dickite, sericite, and diaspore prevail in altered host rocks in the Centralni East deposit. At the Shahumyan deposit, phyllic to argillic alteration with sericite, quartz, pyrite, and dickite is dominant with polymetallic veins, and advanced argillic alteration with quartz-alunite ± kaolinite and dickite is locally developed. The lead isotope composition of sulfides and alunite (206Pb/204Pb = 18.17–18.32, 207Pb/204Pb = 15.57–15.61, 208Pb/204Pb = 38.17–38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallization of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The δ18O values of hydrothermal quartz (8.3–16.4‰) and the δ34S values of sulfides (2.0–6.5‰) reveal a dominantly magmatic source at all three deposits. Combined oxygen, carbon, and strontium isotope compositions of hydrothermal calcite (δ18O = 7.7–15.4‰, δ13C = −3.4−+0.7‰, 87Sr/86Sr = 0.70537–0.70586) support mixing of magmatic-derived fluids with seawater during the last stages of ore formation at Shahumyan and Centralni West. 40Ar/39Ar dating of hydrothermal muscovite at Centralni West and of magmatic-hydrothermal alunite at Shahumyan yield, respectively, a robust plateau age of 161.78 ± 0.79 Ma and a disturbed plateau age of 156.14 ± 0.79 Ma. Re-Os dating of pyrite from the Centralni East deposit yields an isochron age of 144.7 ± 4.2 Ma and a weighted average age of the model dates of 146.2 ± 3.4 Ma, which are younger than the age of the immediate host rocks. Two different models are offered, depending on the reliability attributed to the disturbed 40Ar/39Ar alunite age and the young Re-Os age. The preferred interpretation is that the Centralni West Cu deposit is a volcanogenic massive sulfide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting, before the late Oxfordian. Alternatively, but less likely, the three deposits could represent different mineralization styles successively emplaced during evolution and growth of a magmatic arc during a longer time frame between the Middle and Late Jurassic.
Sericite
Alunite
Argillic alteration
Hypogene
Stockwork
Breccia
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Argillic alteration
Hypogene
Alunite
Sericite
Pyrophyllite
Marcasite
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Argillic alteration
Hypogene
Alunite
Sericite
Authigenic
Marcasite
Sulfide Minerals
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The high-sulfidation system of Cerro Millo is hosted in a Late Miocene andesitic paleo-stratovolcano in the High Andes of southern Peru. Very pronounced advanced-argillic (alunite), silicic, and argillic (kaolinite and smectite) alteration characterizes the central part of the hydrothermal system. Propylitic alteration is developed in a 3 to 4 km wide outer halo. Abundant alunite occurs as hypogene, acicular crystals, and very fine-grained aggregates; the latter formed during near-surface steam-heated overprinting. Hypogene alunite has an Ar–Ar plateau age of 10.8 ± 0.9 Ma (2σ), and is synchronous with the andesitic volcanism (Ar–Ar on biotite: 11.0 ± 0.5 Ma). A second ill-defined alunite age plateau of 8.0 ± 0.9 Ma is probably related to steam-heated overprint and points to major erosion in between both hydrothermal events. Telescoping is also evident by a series of silicified horizons which mark the paleo-groundwater table. These units have elevated mercury, antimony and arsenic levels. Late-phase barite occurs in some structurally controlled advanced-argillic altered envelopes. The hypogene alteration mineralogy points to temperatures at ≤ 250 °C. Hydraulic fracturing and steam-heated overprinting suggest a shallow boiling environment. Slight gold enrichment is observed in the lowermost exposed parts of the system.
Alunite
Argillic alteration
Hypogene
Overprinting
Fumarole
Paragenesis
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The Tiegelongnan deposit in central Tibet presents a typical high‐sulfidation porphyry Cu (Au) mineralization system. Optical microscopy, scanning electron microscopy–energy dispersive spectrometry (SEM–EDS), and laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) were used to understand the mineralization processes. The advanced argillic alteration zone directly contacts the phyllic alteration zone, which is linked to the Early Cretaceous porphyritic intrusions. In this study, the spatial distributions of chief copper sulphides with variable gangue minerals are illustrated. The deep inner phyllic zone typically produced an end‐member assemblage of chalcopyrite–sericite (±dolomite), which trended upwards to produce more bornite–sericite (±siderite ± magnesite), and subsequently, the advanced argillic alteration yielded the representative end‐member assemblage of tennantite–alunite ± (aluminium‐phosphate‐sulphate, namely, APS, ±kaolinite). In the phyllic–advanced argillic transition zone, intensive covellite precipitation with the widespread occurrence of sericite–kaolinite (±APS) assemblage is proposed as an important feature in this study, which is mainly produced on the southwest side of the ore‐body centre. The typical co‐occurrence of chalcopyrite with dolomite at great depths indicates that it was related to a gently acidic fluid containing CO 2 ; however, the pervasive tennantite–alunite association present at shallower levels suggests that fluids with SO 2 were mainly involved and produced stronger acidic conditions. Acidic fluids are demonstrated to highly accelerate water‐rock alteration. Trace element analyses of the copper sulphides reflect that Au concentrations of bornite and tennantite are higher than chalcopyrite and covellite and present small signal variations. Moreover, comparing the presence of micron‐sized Au‐telluride in tennantite under advanced argillic alteration with the nano‐sized native gold in bornite resulting from phyllic alteration suggests that to a certain extent, Au enrichment is affected by the shift of more Te contents in fluid towards shallower levels, which could promote Au precipitation. Additionally, the Se contents of the selected samples were typically concentrated in chalcopyrite, bornite, and covellite, which yielded contents an order of magnitude greater than those in tennantite, indicating the intimate hydrothermal origin of the first three sulphides. Overall, the sulphide–gangue mineral associations and their copper sulphide geochemistry indicate that the hypogene–chalcopyrite of low Au contents from the phyllic zone closely corresponds to the action of gentle acidic fluids with CO 2 , while more acidic fluids with SO 2 are involved in the formation of supergene tennantite with relatively high values of Au and Te in the advanced argillic alteration.
Argillic alteration
Bornite
Alunite
Sericite
Covellite
Hypogene
Stockwork
Chalcocite
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Woodhouseite and svaubergite, previously documented from roughly two dozen localities, have been found with advanced argillic alteration in three hydrothermal ore deposits. These aluminum phosphate-sllfate (APS) minerals are isostructural with alunite (R3m), with the generalized formula RAl3(POdl +r(SO/1-r(OH)6-r. (HzO)r, where R is Ca or Sr, and x is less than 0.5. At Summitville, Colorado, an epithermal gold*opper deposit, svanbergite occurs with hypogene kaolinite and alunite in the upper portions of the deposit, and woodhouseite is observed at deeper levels, where pyrophyllite is locally abundant. In the porphyry-copper deposit at La Granja @eru), woodhouseite occurs witl pyrophyllite and appears to have replaced apatite. The porphyry-copper deposit at La Escondida (Chile) contains woodhouseite-svanbergite solid solutions, some hypogene, others supergene, as judged from textual criteria. Alunite specimens from Summitville, La Escondida, and several other localities investigated in this study contain some APS component, lvith up to 2.41 wt. q0 P2O5 and l.l2wt.tlo SrO, reflecting tie coupled substitution K+ + SO42- = (Ca,Sr)z+ + POo3- io gt. t*t r*ture. The APS phases are considered to form by replacement of apatite in the acidic, sulfate-rich environment ttrat characterizes advauccd argillic alteration. Their occurrence probably has been overlooked in loany areas showing such alteration of apatite-bearing host rocks.
Alunite
Argillic alteration
Hypogene
Pyrophyllite
Supergene (geology)
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Citations (137)