Mineralium Deposita (v.53, #7)

Meta-evaporite in the Carajás mineral province, northern Brazil by Walter Riehl; Alexandre Raphael Cabral (895-902).
Evidence for connecting evaporite-sourced high-salinity fluids with iron-oxide–copper–gold (IOCG) deposits in the Carajás mineral province has solely been based on boron-isotope compositions of tourmaline. Presence of meta-evaporitic rocks remains unrecognised. Here, we report laminated albitite, tourmalinite and banded albite–phlogopite rock, intercepted by exploratory drilling in a clastic metasedimentary sequence. These rocks represent evaporite precursors. Their location in the copper–gold prospects Açaí and Angélica, in the westernmost part of the Carajás mineral province, indicates that (i) evaporite-sourced fluids were regional and (ii) evaporite-bearing metasedimentary sequences may have been an important source of high-salinity fluids and/or sulfur for the IOCG deposits of the Carajás mineral province.

Sulfide minerals host most of the world’s supplies of metals such as Cu, Ni, Co, Zn, Pb, or Mo, and understanding their crystallization, dissolution, and textural evolution is key to understanding the formation of mineral deposits, metal recovery via metallurgy, and their environmental impact. Despite the prominence of hydrothermally formed sulfide minerals, the interpretation of textures in sulfide petrology relies mainly on comparison with results from experiments conducted under dry conditions. Here, we show experimentally that the exsolution of chalcopyrite (CuFeS2) lamellae from a bornite (Cu5FeS4)-digenite (Cu9S5) solid solution (bdss) is a back-reaction, which occurred during low temperature annealing (150 °C) following initial replacement of parent chalcopyrite by bdss at 300 °C. The back-reaction is rapid (days), and its progress is catalyzed by small amounts of fluid present in the porosity within the bdss. This porosity initially results from the formation of bdss via the replacement of parent chalcopyrite by an interface-coupled dissolution-reprecipitation mechanism. During annealing, bdss first breaks down into bornite and digenite via solid-state exsolution. The bornite-digenite assemblage then exsolves chalcopyrite in local patches. We discovered strikingly similar textures in iron oxide copper gold (IOCG) deposits from South Australia; in these samples, the fluid-driven nature of the exsolution reaction is reflected by the fact that the chalcopyrite lamellae propagate around cracks and fractures within bornite. As in our experiments, most of this natural bornite formed via replacement of chalcopyrite. These reactions are controlled by kinetic factors (e.g., relative nucleation rates of Cu-Fe-sulfides; presence of porosity) rather than equilibrium thermodynamics, but result in final assemblages and textures that may not be distinguishable from those evolving from equilibrium processes under dry conditions.
Keywords: Chalcopyrite; Bornite; Mineral replacement; Exsolution; Hydrothermal; Bornite-digenite solid solution

The Triassic gold deposits of the Youjiang Basin, southern China, have been variously correlated to Carlin-style and orogenic gold deposits or classified as a new intermediate deposit type. However, in terms of a multi-scale mineral system approach, they show remarkable similarities to the Tertiary Carlin-type deposits of Nevada and distinct contrasts to orogenic gold deposits. Both the Nevada and Youjiang deposit groups formed in a continent-scale post-orogenic extension event on fragmented continental crust underlain by metasomatized lithosphere. Both form roughly orthogonal deposit trends that subparallel near-orthogonal margins of a continental crustal block, with deposits controlled by gentle anticlines, monoclines or half-horsts and extensional faults, not tight, “locked-up” anticlines, and shear zones. The mineralogy and ore geochemistry of the two groups are similar, with differences consistent with slightly deeper and higher temperature of formation of the older Chinese deposits, commensurate with deeper erosional levels. The Youjiang gold deposits should be classified as Carlin-type, rather than Carlin-like or other terminologies, with their lower gold endowment probably related to a more distal thermal and fluid source than the Nevada Carlin-type deposits.

Mineralized breccia clasts: a window into hidden porphyry-type mineralization underlying the epithermal polymetallic deposit of Cerro de Pasco (Peru) by Bertrand Rottier; Kalin Kouzmanov; Vincent Casanova; Anne-Sophie Bouvier; Lukas P. Baumgartner; Markus Wälle; Lluís Fontboté (919-946).
Cerro de Pasco (Peru) is known for its large epithermal polymetallic (Zn-Pb-Ag-Cu-Bi) mineralization emplaced at shallow level, a few hundred meters below the paleo-surface, at the border of a large diatreme–dome complex. Porphyry-style veins crosscutting hornfels and magmatic rock clasts are found in the diatreme breccia and in quartz–monzonite porphyry dikes. Such mineralized veins in clasts allow investigation of high-temperature porphyry-style mineralization developed in the deep portions of magmatic–hydrothermal systems. Quartz in porphyry-style veins contains silicate melt inclusions as well as fluid and solid mineral inclusions. Two types of high-temperature (> 600 °C) quartz–molybdenite–(chalcopyrite)–(pyrite) veins are found in the clasts. Early, thin (1–2 mm), and sinuous HT1 veins are crosscut by slightly thicker (up to 2 cm) and more regular HT2 veins. The HT1 vein quartz hosts CO2- and sulfur-rich high-density vapor inclusions. Two subtypes of the HT1 veins have been defined, based on the nature of mineral inclusions hosted in quartz: (i) HT1bt veins with inclusions of K-feldspar, biotite, rutile, and minor titanite and (ii) HT1px veins with inclusions of actinolite, augite, titanite, apatite, and minor rutile. Using an emplacement depth of the veins of between 2 and 3 km (500 to 800 bar), derived from the diatreme breccia architecture and the supposed erosion preceding the diatreme formation, multiple mineral thermobarometers are applied. The data indicate that HT1 veins were formed at temperatures > 700 °C. HT2 veins host assemblages of polyphase brine inclusions, generally coexisting with low-density vapor-rich inclusions, trapped at temperatures around 600 °C. Rhyolitic silicate melt inclusions found in both HT1 and HT2 veins represent melt droplets transported by the ascending hydrothermal fluids. LA-ICP-MS analyses reveal a chemical evolution coherent with the crystallization of an evolved rhyolitic melt. Quartz from both HT1 and HT2 veins also contains secondary, low-temperature (~ 300 °C) brine and aqueous fluid inclusions that record the cooling of the system. Both vein types are locally crosscut and/or reopened by a pre-diatreme polymetallic event consisting of pyrite, sphalerite with “chalcopyrite disease,” galena, chalcopyrite, tetrahedrite–tennantite, and minor quartz. LA-ICP-MS analyses of mineral and high-temperature fluid inclusions hosted in HT1 and HT2 veins and in situ secondary-ion mass spectrometry oxygen isotope analyses of vein quartz indicate a magmatic signature for the mineralizing fluids with no major meteoric water input and allow reconstruction of the source and chemical evolution of fluids that formed these porphyry-style veins as snapshots of the early and deep mineralizations at Cerro de Pasco. This detailed study of the porphyry-type mineralization hosted in clasts offers a unique opportunity to reconstruct the late magmatic and early hydrothermal evolutions of porphyry mineralization underlying the world-class Cerro de Pasco epithermal polymetallic (Zn-Pb-Ag-Cu-Bi) deposit.
Keywords: Porphyry; Breccia clasts; Fluid inclusions; Silicate melt inclusions; Hydrothermal quartz; La-ICP-MS; Trace elements; Oxygen isotopes; SIMS; Cerro de Pasco

A genetic link between magnetite mineralization and diorite intrusion at the El Romeral iron oxide-apatite deposit, northern Chile by Paula A. Rojas; Fernando Barra; Martin Reich; Artur Deditius; Adam Simon; Francisco Uribe; Rurik Romero; Mario Rojo (947-966).
El Romeral is one of the largest iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile. The Cerro Principal magnetite ore body at El Romeral comprises massive magnetite intergrown with actinolite, with minor apatite, scapolite, and sulfides (pyrite ± chalcopyrite). Several generations of magnetite were identified by using a combination of optical and electron microscopy techniques. The main mineralization event is represented by zoned magnetite grains with inclusion-rich cores and inclusion-poor rims, which form the massive magnetite ore body. This main magnetite stage was followed by two late hydrothermal events that are represented by magnetite veinlets that crosscut the massive ore body and by disseminated magnetite in the andesite host rock and in the Romeral diorite. The sulfur stable isotope signature of the late hydrothermal sulfides indicates a magmatic origin for sulfur (δ34S between − 0.8 and 2.9‰), in agreement with previous δ34S data reported for other Chilean IOA and iron oxide-copper-gold deposits. New 40Ar/39Ar dating of actinolite associated with the main magnetite ore stage yielded ages of ca. 128 Ma, concordant within error with a U-Pb zircon age for the Romeral diorite (129.0 ± 0.9 Ma; mean square weighted deviation = 1.9, n = 28). The late hydrothermal magnetite-biotite mineralization is constrained at ca. 118 Ma by 40Ar/39Ar dating of secondary biotite. This potassic alteration is about 10 Ma younger than the main mineralization episode, and it may be related to post-mineralization dikes that crosscut and remobilize Fe from the main magnetite ore body. These data reveal a clear genetic association between magnetite ore formation, sulfide mineralization, and the diorite intrusion at El Romeral (at ~ 129 Ma), followed by a late and more restricted stage of hydrothermal alteration associated with the emplacement of post-ore dikes at ca. 118 Ma. Therefore, this new evidence supports a magmatic-hydrothermal model for the formation of IOA deposits in the Chilean Iron Belt, where the magnetite mineralization was sourced from intermediate magmas during the first Andean stage. In contrast, the beginning of the second Andean stage is characterized by shallow subduction and a compressive regime, which is represented in the district by the emplacement of the Punta de Piedra granite-granodiorite batholith (100 Ma) and marks the end of iron oxide-apatite deposit formation in the area.
Keywords: Iron oxide-apatite; Sulfur stable isotopes; Geochronology; Northern Chile

The Wusihe carbonate-hosted Zn-Pb deposit (3.7 Mt. Zn + Pb at a grade of 8.6% Zn and 2.0% Pb) is the largest deposit in the Dadu River Valley district of the Sichuan-Yunnan-Guizhou metallogenic province of southwest China. Three types of orebodies occur: (1) stratiform, banded and lamellar, within dolomite of the Neoproterozoic Dengying Formation; (2) vein type; and (3) breccia type. Four stages of mineralization are distinguished: (i) pyrite stage, (ii) pyrite-pyrrhotite-galena-sphalerite-bitumen stage, (iii) sphalerite-galena stage, and (iv) bitumen-calcite stage. Sphalerite and galena from stages II and III show δ34S ranges from +7.1 to +9.7‰ and +9.1 to +13.1‰, respectively. High-precision in situ lead isotope analyses of sulfides show 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of 37.938 to 38.336, 15.579 to 15.682, and 17.951 to 18.195, respectively, which suggest a mixing of lead from the basement and the host rocks. Rb-Sr isotope analyses for six sphalerite samples of stage II yielded an isochron age of 411 ± 10 Ma (MSWD = 1.4). Combining all available ore geology and geochemical data, together with fluid inclusion data reported previously, we suggest that the Wusihe deposit is a Mississippi Valley-type (MVT) deposit.
Keywords: Rb-Sr isotopic dating; High-precision in situ lead isotopes; Sulfur isotopes; Wusihe MVT Zn-Pb deposit; Yangtze Block

Formation of placer accumulations in fluvial environments requires 103–106 or even greater times concentration of heavy minerals. For this to occur, regular sediment supply from erosion of adjacent topography is required, the river should remain within a single course for an extended period of time and the material must be reworked such that a high proportion of the sediment is removed while a high proportion of the heavy minerals remains. We use numerical modeling, constrained by observations of circum-Pacific placer gold deposits, to explore processes occurring in evolving river systems in dynamic tectonic environments. A fluvial erosion/transport model is used to determine the mobility of placer gold under variable uplift rate, storm intensity, and rock mass strength conditions. Gold concentration is calculated from hydraulic and bedload grain size conditions. Model results suggest that optimal gold concentration occurs in river channels that frequently approach a threshold between detachment-limited and transport-limited hydraulic conditions. Such a condition enables the accumulation of gold particles within the framework of a residual gravel lag. An increase in transport capacity, which can be triggered by faster uplift rates, more resistant bedrock, or higher intensity storm events, will strip all bedload from the channel. Conversely, a reduction in transport capacity, triggered by a reduction in uplift rate, bedrock resistance, or storm intensity, will lead to a greater accumulation of a majority of sediments and a net decrease in gold concentration. For our model parameter range, the optimal conditions for placer gold concentration are met by 103 times difference in strength between bedrock and fault, uplift rates between 1 and 5 mm a−1, and moderate storm intensities. Fault damage networks are shown to be a critical factor for high Au concentrations and should be a target for exploration.
Keywords: Placer deposits; Numerical modeling; Fault damage networks; Gold; Erosion; Fluvial

Selwyn basin area strata contain sedimentary pyrite with Au above background levels when analyzed by laser ablation-inductively coupled mass spectrometry. Hyland Group rocks contain framboidal pyrite contents of 670 ppb Au, 1223 ppm As, and 5.3 ppm Te; the mean of all types of sedimentary pyrite in the Hyland Group is 391 ppb Au, 1489 ppm As, and 3.8 ppm Te. These levels are similar to sedimentary pyrite in host lithologies from major orogenic gold districts in New Zealand and Australia. Comparison of whole rock and pyrite data show that rocks deposited in continental slope settings with significant terrigenous input contain pyrite that is consistently enriched in Au, As, Te, Co, and Cu. Although data are limited, whole rock samples of stratigraphic units containing Au-rich pyrite also contain high Au, indicating that most of the Au is within sedimentary pyrite. Based on geologic characteristics and comparison of pyrite chemistry data with whole rock chemistry, Selwyn basin area strata have the necessary ingredients to form orogenic gold deposits: Au-enriched source rocks, metamorphic conditions permissive of forming a metamorphic ore fluid, and abundant structural preparation for channeling fluids and depositing ore.
Keywords: Pyrite; Shale; Orogenic gold deposits; Selwyn basin; Laser ablation-inductively coupled plasma mass spectrometry; LA-ICPMS

In situ strontium and sulfur isotope investigation of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland by Kirsi Luolavirta; Eero Hanski; Wolfgang Maier; Yann Lahaye; Hugh O’Brien; Frank Santaguida (1019-1038).
The ~ 2.06-Ga Kevitsa mafic-ultramafic intrusion in northern Finland hosts a large disseminated Ni-Cu-PGE deposit. The deposit occurs in the ultramafic olivine-pyroxene cumulates and shows a range in Ni tenors varying from 4–7 wt% (regular ore) to > 10 wt% (Ni-PGE ore). There are also a metal-poor sulfide mineralization (false ore) and contact mineralization that are uneconomic (Ni tenor < 4 wt%). The obtained 87Sr/86Sr(i) values of the Kevitsa ultramafic cumulates are highly radiogenic (> 0.7045) in comparison to the estimated depleted mantle Sr isotope ratio of ~ 0.702 at 2.06 Ga. The sulfur δ 34S values are generally higher than + 2‰, which together with the Sr isotope data imply involvement of crustal material in the genesis of the Kevitsa intrusion and its ores. The 87Sr/86Sr(i) values obtained from the ore-bearing domain of the intrusion show stratigraphic variation and exceed 0.7050, with the maximum value reaching up to 0.7109. In contrast, in rocks around the ore domain, the initial Sr isotope compositions remain more or less constant (0.7047–0.7060) throughout the intrusive stratigraphy. The isotope data suggest that the ore-bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma, whereas the surrounding part of the intrusion experienced a less vigorous emplacement history. No correlation is observed between the strontium and sulfur isotope compositions. This is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the Kevitsa magma chamber. The low level of metals in false ore and the Ni-depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement. Cannibalization of early-formed sulfides by later magma injections may have been important in the formation of the economic ore deposit.
Keywords: Strontium isotopes; Sulfur isotopes; Ni-Cu-PGE sulfide ore; Kevitsa intrusion; Central Lapland greenstone belt

The Yamansu Fe deposit (32 Mt at 51% Fe) in the Eastern Tianshan Orogenic Belt of NW China is hosted in early Carboniferous volcano-sedimentary rocks and spatially associated with skarn. The paragenetic sequence includes garnet-diopside (I), magnetite (II), hydrous silicate-sulfide (III), and calcite-quartz (IV) stages. Pyrite associated with magnetite has a Re-Os isochron age of 322 ± 7 Ma, which represents the timing of pyrite and, by inference, magnetite mineralization. Pyrite has δ 34SVCDT values of − 2.2 to + 2.9‰, yielding δ 34SH2S values of − 3.1 to 2‰, indicating the derivation of sulfur from a magmatic source. Calcite from stages II and IV has δ 13CVPDB values from − 2.5 to − 1.2‰, and − 1.1 to 1.1‰, and δ 18OVSMOW values from 11.8 to 12.0‰ and − 7.7 to − 5.2‰, respectively. Calculated δ 13C values of fluid CO2 and water δ 18O values indicate that stage II hydrothermal fluids were derived from magmatic rocks and that meteoric water mixed with the hydrothermal fluids in stage IV. Some ores contain magnetite with obvious chemical zoning composed of dark and light domains in BSE images. Dark domains have higher Mg, Al, Ca, Mn, and Ti but lower Fe and Cr contents than light domains. The chemical zoning resulted from a fluctuating fluid composition and/or physicochemical conditions (oscillatory zoning), or dissolution-precipitation (irregular zoning) via infiltration of magmatic-hydrothermal fluids diluted by late meteoric water. Iron was mainly derived from fluids similar to that in skarn deposits.
Keywords: Re-Os geochronology; Trace elements; Magnetite; Dissolution-reprecipitation; Yamansu Fe deposit