Low-sulfidation Epithermal Deposit Research Articles

Sm–Nd fluorite dating of Proterozoic low-sulfidation epithermal Au–Ag deposits and U–Pb zircon dating of host rocks at Mallery Lake, Nunavut, Canada

The Mallery Lake area contains precious metal-bearing quartz–chalcedony stockworks that are pristine examples of ancient low-sulfidation epithermal deposits. Fluorite extracted from these epithermal deposits define a Sm–Nd errorchron age of 1434 ± 23 Ma mean square of weighted deviates (MSWD) = 4.8. This date is interpreted to have age significance because (1) a simple linear trend does not exist between the 143Nd/144Nd ratios of the fluorite with respect to their 1/Nd concentrations as would be expected for mixing of two geochemical end members; (2) microthermometric studies indicate that the fluorite analysed in this study has an intimate association with a single high-salinity, calcic brinal fluid; and (3) the age determined from seven fluorite samples extracted from a single outcrop location yielded an identical age result (1434 ± 60 Ma; MSWD = 5.5) compared to the fluorite composite. Rhyodacites of the Pitz Formation and syenites from the Nueltin suite (intrusive equivalent to the rhyodacites) are the youngest volcanic–plutonic rocks that are observed in outcrop in the Mallery Lake area, and they were dated by U–Pb zircon analysis at 1760 ± 43 Ma and 1755.4 ± 1.8 Ma, respectively. The ~320 million year age difference between the epithermal deposits and the hosting rhyodacitic flows suggests that the epithermal stockworks may have formed by a regional hydrothermal event unrelated to this earlier Paleoproterozoic volcanic activity. Uranium deposits in the Thelon and Athabasca basins, to the northwest and south of the Baker Lake Basin, were determined to have similar ore emplacement ages with no evident heat source.

Fluid origin of the Ixtacamaxtitlán hydrothermal deposits, Puebla State, Mexico

The hydrothermal deposit of Ixtacamaxtitlán (Puebla, Mexico) is made up by a succession, from bottom to top, of quartzveins and stockwork enclosed in a porphyritic subvolcanic body, a kaolinitized rhyolitic tuff and a layered opal deposit. This vertical arrangement coupled with the distribution of the different alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit. The fluid inclusion study carried on the veins and the stockwork along with the stable isotopic analyses performed on the kaolinitized bodies helped us to propose two major hydrothermal events that occurred in the area: an early event, characterized by hot, hypersaline fluids (up to 280 °C and 36 wt.% NaCl eq.) closely associated with a potassic alteration episode; and a late event, distinguished by cooler and dilute fluids (up to 150 °C and 4 wt.% NaCI eq.), associated with propylitic and quartz-sericite alterations at depth and acid-sulfate alteration close to the paleosurface due to steam-heated phreatic waters. Post-trapping changes found affecting the primary fluid inclusions (hook-shaped morphologies) suggest that there was an uplifting period between the two stages.

Lode Gold and Epithermal Deposits of the Mallina Basin, North Pilbara Terrain, Western Australia

The Mallina basin, in the central west part of the North Pilbara terrain, is one of the more mineralized parts of the Pilbara craton. Three styles of Au mineralization are present in the region: (1) lode Au deposits associated with sericite-carbonate-pyrite alteration assemblages, (2) lode Au deposits associated with pyrophyllite-bearing alteration assemblages, and (3) lode Sb-Au deposits. Gold resources have been identified at the Withnell and Camel deposits just to the north of the east-trending Mallina shear zone. East-trending mineralized zones at the Withnell deposit are typical of turbidite-hosted lode Au deposits in that they are late tectonic (syn- to late- local D3) and associated with extensive carbonate-sericite alteration. Gold mineralization occurs in planar quartz carbonate veins and with pyrite and arsenopyrite in the host rocks to the veins. Fluid inclusion analyses indicate mainly low-salinity, carbonic inclusions with moderate homogenization temperatures. The lode Sb-Au deposits have a similar structural setting and fluid characteristics. At the Camel 2 deposit Au is associated with syn- to late-D3 pyrophyllitic shear zones that have overprinted an earlier carbonate alteration assemblage. Structural relationships and limited Pb isotope data suggest that the lode Au mineralization in this district formed at about 2900 Ma. In addition to the lode Au and Sb-Au deposits, north-northwest–trending quartz veins have textures typical of low-sulfidation epithermal deposits. The lithogeochemistry of samples collected from these veins indicates local enrichment of Au, As, Sb, Ag, Te, and Hg, which is consistent with an epithermal origin. Based on geologic relationships and Pb isotope data, these deposits are interpreted to have formed in the Mallina basin at ca. 2750 Ma. If they are true epithermal deposits, then they are among the oldest known. Their presence suggests that preservation, not Archean age, is the key control on the temporal distribution of epithermal deposits.

Supergene Alteration of Primary Ore Assemblages from Low-Sulfidation Au-Ag Epithermal Deposits at Pongkor, Indonesia, and Nazareño, Perú

The textures and mineralogical assemblages of high-grade ore samples from the low-sulfidation Au-Ag epithermal deposits of Pongkor (2 Ma; West Java, Indonesia) and Nazareno (10 Ma; Cordillera Shila, southern Peru) were studied to investigate processes related to post-depositional alteration of primary mineral parageneses. Both deposits are characterized by a late primary high-grade base and precious metal stage composed of chalcopyrite-sphalerite-galena-pyrite–polybasite-pearceite–tennantite-tetrahedrite–electrum. Minerals of the polybasite-pearceite series, the main Ag carriers, were found to be transformed into acanthite. Acanthite was also found locally in association with native silver, as a replacement product of base metal sulfides, as veinlets, alternating rhythmically with supergene iron oxyhydroxides replacing pyrite, as well as associated with barite, iron oxyhydroxides and covellite as boxwork fillings. Primary electrum grains with a composition of about Au0.6Ag0.4 are partially replaced by Au-Ag sulfide, locally identified as uytenbogaardtite (Ag3AuS2), and/or by acanthite, with both of the replacing minerals commonly associated with vermicular electrum of a greater fineness (up to Au0.8Ag0.2). The occurrence of the replacement textures near the present-day surface and the associated secondary minerals, strongly suggests a supergene origin for the Au and Ag sulfide phases. The comparison of Pongkor and Nazareno also suggests that supergene remobilization of Au and Ag is geochemically significant and can occur in widely differing morphological and climatic contexts.

Fluid evolution in the Baia Mare epithermal gold/polymetallic district, Inner Carpathians, Romania

The Baia Mare district (Romania) represents a complex segment of the Neogene/Quaternary, subduction-related, volcanic chain of the Carpathians. In this province, the calc-alkaline volcanism and associated mineralization are closely related to a major strike-slip fault and an underlying pluton. The ore deposits of the district have many features typical of low-sulfidation (or adularia-sericite) epithermal deposits. The combination of fluid-inclusion microthermometry, Raman spectroscopy, bulk crush-leach analysis, laser ablation-optical emission spectrometry (LA-OES) of individual fluid inclusions and stable isotope data has been used to characterize the ore-forming solutions involved with gold/polymetallic mineralization in three of the largest deposits. Ore fluids at Baia Sprie, Cavnic and Sasar deposits had temperatures of 320 to 150 °C and salinities in the range 0–21 wt% NaCl equiv. During phreatomagmatic activity, fracturing and periods of vein dilation, the pressure fluctuations may have caused boiling. The hydrothermal fluids were Na–K–Li–Ca chloride solutions with lesser amounts of SO4 and F. The Na/K ratios and the stable isotope compositions are consistent with equilibration of the fluids with country rocks at progressively lower temperatures. The halogen ratios (Br/Cl×103 in the range 0.13–1.21), together with sulfur isotopic systematics (δ34S=0.3–5.2 per mil), are consistent with a dominantly magmatic origin of the solutes, and probably of the ore fluids themselves.

Isotopic and fluid-inclusion constraints on the formation of polymetallic vein deposits in the central Argentinian Patagonia

The lead isotope compositions of galena and the fluid-inclusion systematics of nine barite-bearing polymetallic (Au, Ag, Pb, Zn) deposits of the central Argentinian Patagonia (Chubut and Rio Negro provinces) have been investigated to constrain the compositions and sources of the mineralizing fluids. Most of the deposits occur as veins, with less common wall-rock disseminations and/or stockworks, and are low-sulfidation epithermal deposits hosted in Jurassic volcanic rocks. Fluid-inclusion homogenization temperatures (Th) from quartz and sphalerite from the deposits fall within the range of 100–300 °C, with the highest measured average temperatures for the most eastern deposits (Mina Angela – 298 °C; Canadon Bagual – 343 °C). The salinities of the hydrothermal fluids at all deposits were low to moderate (≤10.4 equiv. wt% NaCl). Three groups of ore deposits can be defined on the basis of 206Pb/204Pb ratios for galena and these show a general decrease from west to east (from 18.506 to 18.000). The central Argentinian Patagonia deposits have distinctly less radiogenic lead isotope compositions than similar deposits from Peru and Chile, except for the porphyry copper deposits of central and southern Peru. Galena from the Mina Angela deposit is characterized by very low radiogenic lead isotope compositions (18.000<206Pb/204Pb<18.037 and 38.03<208Pb/204Pb<38.09) and reflects interaction with Precambrian basement. The geographic trend in lead isotope compositions of both galena and whole rocks indicates a crustal contribution which increases eastwards, also reflected in the strontium-neodymium isotope systematics of the host lavas. Finally, due to the lack of precise age determinations for the central Patagonian polymetallic deposits, a potential link with Andean porphyry copper systems remains an open question.

PRECIPITATION OF GOLD IN A LOW-SULFIDATION EPITHERMAL GOLD DEPOSIT: INSIGHTS FROM SUBMILLIMETER-SCALE OXYGEN ISOTOPE ANALYSIS OF VEIN QUARTZ—A DISCUSSION

Sir: Hayashi et al.’s (2001) study of vein quartz from the Hishikari mine has provided a good test of the model of gold deposition by boiling that we put forward in a paper (Bogie and Lawless, 1999) on the significance of the mineralogical and textural variation in low-sulfidation epithermal gold deposits. Our model is based on the textural evidence for the silica supersaturation at elevated temperatures in epithermal gold deposits that is observed within bands of rhythmically banded gold ore. This is exactly the material that is shown in figure 5 of Hayashi et al. (2001), although the authors describe the band of recrystallized chalcedony that typically starts off each rhythmic cycle as bladed quartz. Other workers, e.g., Faure et al. (1998), describe this material at Hishikari as chalcedony. We and earlier workers (Fournier, 1985; Saunders, 1994) interpret recystallized chalcedony—and in some cases, quartz—that retains ghosts of colloform textures as having initially been amorphous silica that has then recrystallized to chalcedony or quartz, as the case may be. This point is supported by the work of Faure et al. (1998), who found excess water in chalcedonic quartz at Hishikari that they interpret to be the result of its initially depositing as a hydrous gel. In …

Inclusions fluides et isotopes du soufre du gisement Cu–Au de Valea Morii (monts Apuseni, Roumanie) : un télescopage porphyre–épithermal neutre ?

Abstract The Valea Morii Cu–Au ore deposit (Apuseni mountains, Romania) is characterised by a spatial association of a porphyry copper (Cu±Au) with a low-sulfidation epithermal deposit (Au). Microthermometric and sulphur isotopic data, obtained on both porphyry and epithermal veins, express a telescoping between these two types of mineralization, which can be explained either by an erosion phenomenon during the hydrothermal activity or by a collapse of the upper part of the hydrothermal system.

Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO2), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl2-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields δ18O values from 8.3 to 14.5‰ (average=10.9±1.7‰ [1σ], n=30), whereas δ18O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0±0.9‰, n=12). Calculated δ18OH2O values for the dilute mineralizing fluid from B veins range from –2.6 to 0.2‰ (average=–0.8±0.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower δ18O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from δDFI=–56 to –134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.

Epithermal Au-Ag-Te Mineralization, Acupan, Baguio District,Philippines: Numerical Simulations of Mineral Deposition

In this study, we discuss the effects of cooling, boiling, fluid mixing, and water-rock interaction on a low-sulfidation chloride water. Our water composition is derived from fluid inclusion and mineralogical studies of the Acupan gold mine, a large gold-silver-tellurium-bearing low-sulfidation epithermal deposit in the Philippines. Our numerical modeling results show that a single mineralizing water (300°C, 0.5 wt ‰ NaCl + KCl, 0.41 m CO 2 ) will evolve along different reaction pathways in response to different physicochemical processes, and that these pathways are difficult to predict intuitively in many cases. Acidity and redox can evolve dramatically and in different directions, with boiling resulting in oxidation and pH increase, cooling resulting in pH decrease at a relatively constant sulfate/sulfide ratio, and mixing with sulfate-bearing ground waters causing oxidation and acidification. Based on the correlation of predicted and observed ore and gangue minerals, boiling is concluded to have resulted in the deposition of most of the precious and base metals at Acupan. Continuous boiling, boiling with intermittent gas loss, and throttling probably all occurred at various times during the evolution of the hydrothermal system. The loss of gases during boiling (e.g., H 2 S, H 2 Te, Te 2 ) enhanced electrum and base metal sulfide deposition and inhibited the precipitation of hessite and calaverite. Mixing of low-temperature ground waters with the high-temperature chloride water resulted in mineral assemblages that are similar to those observed in shallow levels of the mine and in deep-level, late-stage barren vein fill. Mixing with ground water could account for the observed transition from adularia-carbonate vein assemblages in deep mine levels to sericite-bearing assemblages in shallow levels. Late-stage anhydrite could have formed via mixing with or heating of near-surface ground waters. We predict tellurium to be transported preferentially in a gas phase. Because tellurium solubilities are predicted to be low in auriferous chloride waters, telluride and native tellurium deposition in low-sulfidation environments may result from condensation of magmatically derived H 2 Te (g) and Te 2(g) into deep-level chloride waters. The minor amount of tellurium that dissolves into chloride waters will be deposited effectively by cooling or fluid mixing. Aqueous tellurium will partition strongly into the gas phase in boiling low-sulfidation systems and could precipitate via condensation into lower temperature ground waters. This could lead to vertical zonation of electrum and tellurium-bearing minerals, which may be of significance to mineral exploration.

Metamorposed Archean epithermal Au-As-Sb-Zn-(Hg) vein mineralization at the Campbell Mine, northeastern Ontario

The Campbell mine and the adjacent Red Lake mine are developed in an Archean gold deposit hosted within volcanic rocks of the Red Lake greenstone belt, situated in the western part of the Uchi subprovince of the Superior province. Geochronological evidence suggests that the Campbell-Red Lake deposit formed after volcanism but prior to late felsic plutonism and regional metamorphism, between 2722 and 2710 Ma. The orebodies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults.Complex, superimposed phases of wall-rock alteration occurred prior to the emplacement of main stage veins and were controlled by both primary and secondary (contact, fault, and fracture induced) permeability in mafic and ultramafic rocks as well as secondary permeability in rhyolite and diorite. Widespread pervasive carbonatization and potassic (biotite) alteration with distal chloritic alteration is well developed in basalt and ultramafic rocks. In basalt, this alteration was in turn partly overprinted by silicification and aluminous alteration consisting of zoned mineral assemblages of quartz-andalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garnet-cummingtonite assemblages occurring in flanking chloritic zones. Similarly, zoned aluminous mineral assemblages also occur in diorite and rhyolite. In ultramafic rocks, the aluminous alteration is represented by sericite-fuchsite-cordierite assemblages, which grade out toward chlorite-anthophyllite-cordierite assemblages. This zoning appears to reflect the bulk composition of zoned premetamorphic aluminous (argillic) alteration.Main-stage carbonate (dolomite to ankerite)-quartz veins exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures typical of veins which formed in a near-surface environment. Fissure veins occur within fault zones themselves, whereas large snowbank (banded carbonate-quartz) veins formed in dilatant sites associated with a releasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zones occur in ultramafic rocks adjacent to main-stage vein sites in basalt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal chloritic alteration as the hydrothermal system collapsed inward. The main-stage veins were then overprinted by silicification + or - tourmaline within breccias and veinlet stockworks.Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, and sulfide cemented breccias which cut silicified zones in veins and wall rocks. Native gold occurs on its own or with sulfides in fractures cutting silicification. Arsenopyrite is commonly associated with gold and occurs in ore zones with pyrite, pyrrhotite, and magnetite. Stibnite and sphalerite commonly occur in high-grade zones. Mineralized zones hosted in fissure veins and wall-rock replacement zones tend to be narrow (<1 m wide) and in narrow fault zones. Mineralization associated with brecciated snowbank veins are typified by multiple brecciation and fracturing events and are formed in dilatant sites at a high angle to fault zones. Mineralized zones are cut by late hydrothermal breccia dike and pipelike bodies as well as late quartz-carbonate veins and faults.The zoned aluminous alteration, open-space-filling textures of the main-stage veins, sheeted veinlet zones and stockwork fracturing, multiple phases of hydrothermal breccias, and anomalous Au-Ag-As-Sb-Hg-Zn-K at the Campbell mine are characteristics similar to those of Phanerozoic low sulfidation epithermal deposits. The mineralized environment has been deformed (flattened and stretched) and metamorphosed to middle to upper greenschist facies during the Kenoran orogeny (ca. 2650 Ma).