ALBERT

Description: The large Cerro de Pasco Cordilleran base metal deposit in central Peru is the result of three successive mineralizing stages comprising both low- and high-sulfidation mineral associations: (A) several pyrrhotite pipes grading outward to sphalerite and galena replacement bodies, (B) a massive, funnel-shaped pyrite-quartz replacement orebody, and (C) E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies. This superposition of hydrothermal events leads to complex replacement textures and crosscutting relationships. A detailed study of the textures and mineral composition of the up to 15-m-wide replacement front existing between the pyrrhotite pipes and the pyrite-quartz body allows for clarification of the relative chronology of the hydrothermal events. The results show that, in contrast to previous interpretations, the emplacement of the pyrrhotite pipes and their Zn-Pb mineralized rims precedes that of the pyrite-quartz body. The replacement textures affecting pyrrhotite and arsenopyrite and the nature of the newly formed minerals have been used as a qualitative way to track the evolution of f S 2 , f O 2 , and pH of the mineralizing fluids. Two steps of pyrrhotite replacement have been recorded. The first one takes place under moderate acidity and relatively reduced to moderately oxidized conditions and is marked by replacement of pyrrhotite by euhedral nonporous pyrite. The second step occurs under more acidic and oxidized conditions and is characterized by replacement of pyrrhotite by porous marcasite and replacement of arsenopyrite by pyrite. Subsequently, marcasite is partly replaced by fine-grained euhedral nonporous pyrite. LA-ICP-MS trace element analyses of the replaced pyrrhotite and arsenopyrite and of the newly formed marcasite and pyrite support dissolution-reprecipitation as the main mechanism for replacement. Positive correlations between some of the elements (e.g., Pb-Sb, Pb-Ag) are indicative of the possible presence of nanoscale solid inclusions as main carriers for those elements; however, coupled substitutions and incorporation of some of the elements at a ppm level into the pyrite and marcasite structures cannot be excluded. The obtained As, Sb, Pb, and Bi values in pyrite are systematically higher than published data of pyrite in epithermal and porphyry systems. Nature and trace element content of the newly formed minerals yield information on the physicochemical conditions during their precipitation, the initial trace element content of replaced minerals, and the subsequently dissolved neighboring phases. The results show that the metal concentration of the fluid is locally influenced by the composition of the dissolved minerals. This study leads to a simpler interpretation of the fluid evolution than previously proposed, with a progressive increase of f S 2 , f O 2 , and pH as a result of decreasing wall-rock buffering during the three successive mineralizing stages at Cerro de Pasco.

Description: Ten sphalerite separates isolated from mineralized samples in proximal and distal positions relative to the proposed main feeder fault systems at the Laisvall deposit were used to obtain an absolute age determination of this world-class Pb-Zn deposit hosted by autochthonous Ediacaran to Lower Cambrian sandstone and located currently along the erosional front of the Scandinavian Caledonides in northern Sweden. Residue and leachate fractions of each separate were obtained using the crush-leaching technique. All samples correspond to sphalerite formed using reduced sulfur derived from thermochemical sulfate reduction, three of them from disseminated ore in the Lower Sandstone, two from the disseminated ore in the Upper Sandstone, and five from steeply dipping galena-sphalerite-calcite veinlets interpreted in previous works as remobilization of disseminated ores. The isotope dilution-thermal ionization mass spectrometry (ID-TIMS) data yield an overall complex Rb-Sr isotope pattern with two distinct trends in the 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr isochron diagram. The three sphalerite residues of disseminated mineralization from the Lower Sandstone orebody show Rb-Sr isotope systematics indicative of undisturbed primary precipitates, and yield an isochron model age of 467 ± 5 Ma (mean square weighted deviation, MSWD, 1.4). Since the isochron is based on three points, the obtained age is to be considered as preliminary. Yet, the obtained age is fully consistent with geologic evidence reported by previous authors and pointing to Middle Ordovician timing of ore formation. The ID-TIMS data were complemented by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on the same sphalerite samples. The data support the hypothesis that the measured ID-TIMS Rb and Sr contents in these sphalerite residues are held in the sphalerite structure itself and are not related to micro-inclusions. The most viable hypothesis, in agreement with published work, is that during rapid growth, sphalerite may incorporate Rb and Sr ions from the hydrothermal fluids in its structure, most probably in octahedral voids. By contrast, the second trend in the 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr space defined by most other sphalerite residues and corresponding inclusion fluid leachates from the Upper Sandstone orebody and the veinlet samples is too steep to account for a realistic isochron age determination. This steep linear trend is interpreted to represent a postmineralization disturbance involving fluids rich in Sr. This disturbance of the Rb-Sr isotope system is consistent with the presence of the steeply dipping galena-sphalerite-calcite veinlets and the fact that the Upper Sandstone is, in places, tectonically disrupted because of its proximity to the basal Caledonian décollement. The attempt to date the Granberget deposit, located in tectonically disrupted allochthonous units inside the Caledonian orogen, failed because the Rb-Sr isotope systematics of the three analyzed sphalerite samples are also disturbed. The obtained Middle Ordovician (467 ± 5 Ma) mineralization age at Laisvall can be interpreted as a far-field foreland response to an early Caledonian arc-continent collision and the subsequent development of a foreland basin. Basinal brines formed in the foredeep of the orogen could be conveyed cratonward, interact with permeable Baltica crystalline basement rocks, and resurge as metal-bearing fluids in sandstone at Laisvall along reactivated Paleoproterozoic crystalline basement faults. Mixing of metal-bearing brines with hydrocarbon and H 2 S-rich fluids in Ediacaran to Lower Cambrian sandstone may explain the initial Sr isotope signature ( 87 Sr/ 86 Sr = 0.715900 ± 60) of the isochron intersect.

Description: This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earth's crust, from low-temperature aqueous solution to supercritical hydrothermal-magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl 2 – , AuHS, and Au(HS) 2 – complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermodynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations.

Description: Porphyry-related base metal vein and replacement mineralization (i.e., Cordilleran polymetallic mineralization) in the Morococha district, central Peru, is part of a large magmatic-hydrothermal system associated with the emplacement of several late Miocene porphyry intrusions and formation of important Cu-Mo mineralization. Zn-Pb-Ag-Cu veins overprint the giant Toromocho porphyry Cu-Mo deposit in the center of the district and display a typical concentric base metal zonation (Cu -〉 Zn, Pb -〉 Ag) covering an area of approximately 50km 2 . A detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal-rich, intermediate-density porphyry-type fluids. In early stages of Cordilleran base metal vein formation, fluid inclusions have low salinities of ~2 to 5 wt % NaCl equiv, CO 2 contents of 3 to 10mol %, and homogenization temperatures (T h ) of 380° to 340°C. They are similar to intermediate-density fluid inclusions trapped in a milky quartz vein predating Cordilleran polymetallic mineralization, with similar low salinities (3.0–3.8 wt % NaCl equiv) and low CO 2 contents (6.5–8 mol %), but higher T h of ~420° to 410°C. During cooling of the intermediate-density fluids from 400° to 300°C, the lithostatic pressure regime changed to a hydrostatic one. The fluids underwent pressure drop as well as phase separation (i.e., unmixing) and lost most of their CO 2 . They acquired moderate salinities, in some cases intermediate (~up to 16 wt % NaCl) to brine compositions. However, the bulk of the magmatic fluid retained low salinity while it continued to cool under open-system conditions and precipitated tennantite-tetrahedrite, chalcopyrite, enargite, sphalerite, and galena. Upon cooling below 270°C, the fluids deposited abundant rhodochrosite and quartz, while following the boiling curve toward lower P-T conditions. These data record an evolution of mineral precipitation from deep (minimum depth of 2–1.5 km) to shallow environments (300–800 m). Oxygen, hydrogen, and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage. Copper (5,000–18,000 μ g/g), sulfur (up to 12,000 μ g/g), and iron (2,100–6,000 μ g/g) concentrations in the intermediate-density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate-density inclusions of the early Cordilleran base metal veins. The base metals Zn, Pb, and Mn have comparable concentrations between 100 and 1,000 μ g/g for both types of fluid. These findings suggest that the fluids identified in Cordilleran polymetallic veins are compositionally similar to the porphyry-type fluids and could have derived from the latter after precipitation of Cu- and Fe-bearing sulfides in a deeper porphyry environment. The new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry-centered districts (e.g., Bingham, Butte) and show that both porphyry and polymetallic mineralization can precipitate from similar magmatic-hydrothermal fluid pulses.

Description: Porphyry-related Cordilleran polymetallic mineralization in the Morococha district, central Peru, displays a deposit-scale base metal zonation, a common feature observed in many ore deposits of this type. The distinct pattern of zonation consists of Cu-rich ores in the core area and increasingly Zn-, Pb-, and Ag-rich ores in more distal veins and replacement orebodies. We investigated the changing composition of key ore minerals throughout these zones. They are important indicators for the conditions of mineral precipitation and evolving hydrothermal fluid. The mineral species stannoidite Cu 8 (Fe,Zn) 3 Sn 2 S 12 , vinciennite Cu 10 Fe 4 Sn(As,Sb) S 16 , and tennantite-tetrahedrite have variable Cu content and Fe 2+ /Fe 3+ values between the different base metal zones. Controlled by coupled substitution mechanisms, they show Cu-excess and predominantly ferric Fe compositions in the central areas (Cu and Zn-Cu zones), whereas stoichiometric Cu values and mixed Fe 3+ and Fe 2+ compositions are typical in the more distal zones (Zn-Pb-Ag and Ag-Pb). These compositional changes are interpreted to reflect decreasing oxidation state of the cooling and evolving fluid and increasing reaction with the host rock toward outer parts of the deposits. Initially high Cu, Fe, and S values in the fluid decrease with precipitation of Cu- and Fe-sulfides. As result of decreasing oxidation state and Fe and Cu values in the fluid, Fe in tennantite-tetrahedrite is increasingly substituted by competing Zn, and Cu by Ag. These data show that compositions of major and even minor ore-forming minerals reflect metal content, oxidation and sulfidation state, and degree of buffering of an evolving hydrothermal fluid by the host rock on a district-scale.

Description: Hydrothermal ore deposits are large geochemical anomalies of sulfur and metals in the Earth's crust that have formed at 〈1 to ~8 km depth. Sulfide minerals in hydrothermal deposits are the primary economic source of metals used by society, which occur as major, minor and trace elements. Sulfides also play a key role during magmatic crystallization in concentrating metals that subsequently may (or may not) be supplied to hydrothermal fluids. Precipitation of sulfides that themselves may have little economic value, like pyrite, may trigger the deposition of more valuable metals (e.g. Au) by destabilizing the metal-bearing sulfur complexes. We review why, where and how sulfide minerals in hydrothermal systems precipitate.

Description: Physical properties and compositions of ore-forming fluids in magmatic-hydrothermal systems have been mostly investigated by conventional fluid inclusion studies in transparent gangue minerals that are assumed to be co-genetic with the mineralization. However, ore-precipitating fluids can be directly studied by analyzing fluid inclusions in opaque ore minerals, such as wolframite, stibnite, pyrite, and enargite, by using near-infrared (NIR) petrography and microthermometry in combination with laser ablation-inductively coupled plasma-mass spectrometry analysis of individual inclusions. Although results of NIR fluid inclusion studies of ore minerals were first published in 1984, the technique is still not commonly used in fluid inclusion research due to a number of limitations related to the analytical equipment used, sample preparation, and NIR mineral transmittance.In this contribution, we present new data on the applicability of NIR fluid inclusion studies of ore minerals (pyrite, stibnite, enargite, and wolframite) from a series of magmatic-hydrothermal systems, according to their chemical composition and physical behavior during microthermometry. Our results reveal strong correlation between NIR transmittance and high trace element (Co, Ni, Cu, As) content in pyrite, enargite (Fe, Bi), and wolframite (Sc, V, Fe). Despite this, the restricted distribution of these elements in oscillatory and sector zoning has allowed observation of NIR mineral features and fluid inclusions. Energy absorption of opaque minerals, either as light energy during microscopy or as thermal conductive energy during fluid inclusion microthermometry, presents a second limitation for NIR fluid inclusion studies. Our results confirm the relevance of fluid inclusion studies in ore minerals by combining NIR microscopy and microthermometry. Despite some limitations due to trace element composition of the host mineral and its physical behavior at high temperature, a successful NIR fluid inclusion study can be performed on some ore minerals that are opaque in the visible light range, allowing the direct study of hydrothermal ore-forming fluids.