Mineralium Deposita (v.50, #7)
Epithermal paleosurfaces by Richard H. Sillitoe (767-793).
Many active volcanic-hydrothermal and geothermal systems are characterized by distinctive surface and near-surface landforms and products, which are generated during discharge of a spectrum of fluid types under varied conditions. Remnants of most of these products are preserved in some of their less-eroded, extinct equivalents: epithermal deposits of high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) types. Steam-heated alteration occupying vadose zones and any underlying silicified horizons formed at paleogroundwater tables characterize HS, IS, and LS deposits as do hydrothermal eruption craters and their subaerial or shallow sub-lacustrine breccia aprons and laminated infill. Although rarely recognized, HS, IS, and LS systems can also contain finely laminated, amorphous silica sediments that accumulated in acidic lakes and mud pots and, exclusive to HS systems, in hyperacidic crater lakes. In contrast, silica sinter and more distal carbonate travertine are hot spring discharge products confined mainly to LS and IS settings, as both form from near-neutral-pH liquids. Hydrothermal chert deposition and sediment silicification can take place in shallow, lacustrine rift settings, also largely restricted to LS and IS deposits. These surface and near-surface hydrothermal products are typically metal deficient, although mercury concentrations are relatively commonplace and were formerly exploited in places. Nonetheless, sinters, hydrothermal eruption craters, and silicified lacustrine sediments may contain anomalously high precious metal values; indeed, the last of these locally constitutes low-grade, bulk-tonnage orebodies. The dynamic nature of epithermal paleosurfaces, caused by either syn-hydrothermal aggradation or degradation, can profoundly affect deposit evolution, leading to either eventual burial or telescoping of shallower over deeper alteration ± precious metal mineralization. Formational age, tectonic and climatic regime, hydrothermal silica content and texture, and post-mineralization burial history combine to determine the preservation potential of paleosurface products. Proper identification and interpretation of paleosurface products can facilitate epithermal precious metal exploration. Proximal sinters and hydrothermal eruption craters may mark sites of concealed epithermal mineralization, whereas paleogroundwater table silicification and steam-heated blankets can be more widely developed and, hence, less diagnostic. Epithermal precious metal deposits may immediately underlie paleosurface features but are commonly separated from them by up to several hundred vertical meters, especially in the case of IS deposits. Furthermore, the tops of concealed, particularly IS epithermal orebodies in any particular district, irrespective of whether or not paleosurface features are preserved, can also vary by several hundred vertical meters, thereby imposing an additional exploration challenge. Precious metal contents of paleosurface products are unreliable but nonetheless potentially useful guides to concealed deposits. However, sub-paleosurface geochemical anomalism, particularly for arsenic and antimony, may indicate proximity to subjacent ore.
Keywords: Epithermal deposits; Precious metals; Paleosurfaces; Hot springs; Exploration
Geochemistry of magnetite from Proterozoic Fe-Cu deposits in the Kangdian metallogenic province, SW China by Wei Terry Chen; Mei-Fu Zhou; Jian-Feng Gao; Ruizhong Hu (795-809).
Fe-Cu deposits in the Kangdian Fe-Cu metallogenic province, SW China, are hosted in Paleoproterozoic meta-volcanic-sedimentary sequences and are spatially associated with coeval mafic intrusions. Several well-known examples are the giant Lala, Dahongshan, and Yinachang deposits. They have a common paragenetic sequence of an early Fe-oxide stage associated with sodic alteration and a late Cu-sulfide stage associated with potassic-carbonate alteration. Magnetite dominates the Fe-oxide stage of these deposits but is also present in the Cu-sulfide stage of the Lala deposit. This study uses trace element compositions of magnetite to examine the nature and origin of the ore-forming fluids. The magnetite has variable concentrations of Ti, Al, Mg, Mn, Si, V, Cr, Ca, Co, Ni, Sc, Zn, Cu, Mo, Sn, and Ga, which are thought to have been controlled mainly by fluid compositions and/or intensive parameters (e.g., temperature and oxygen fugacity (fO2)). Fluid-rock interaction and coprecipitating mineral phases appear to be less important in controlling the magnetite compositions. Magnetite grains in the Fe-oxide stage of the Lala and Dahongshan deposits have comparable trace element compositions and were likely precipitated from chemically similar fluids. High Ni contents of magnetite in both deposits, coupled with previous isotopic data and the fact that the two deposits are spatially associated with coeval mafic intrusions, strongly suggest that the ore-forming fluids were genetically related to the mafic magmas that formed the intrusions. Magnetite grains in the Fe-oxide stage of the Yinachang deposit have much lower V and Ni but higher Sn and Mo contents than those of the Lala and Dahongshan deposits and are thus thought to have precipitated from more oxidized and Mo-Sn-rich fluids that may have evolved from relatively felsic magmas. Magnetite grains from the Cu-sulfide and Fe-oxide stages of the Lala deposit are broadly similar in composition, but those in the Cu-sulfide stage have slightly higher Cu, Zn, and Mn and are thought to have crystallized from relatively low-temperature and Cu-Zn-Mn-rich fluids evolved from the fluids of the early Fe-oxide stage. Our results show that magnetite from the Fe-Cu deposits in the Kangdian Province, banded iron formation, Fe skarn deposits, diabase-hosted hydrothermal Fe deposits, and magmatic deposits has significantly different compositions. We propose that covariations of Co-Ni, Zn-Sn, and Co/Ni-Mn can be used to effectively discriminate different deposit types.
Keywords: Magnetite; Trace element composition; LA-ICP-MS; IOCG deposit; Kangdian metallogenic province, SW China
Characteristics of Cu isotopes from chalcopyrite-rich black smoker chimneys at Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau basin by H. A. Berkenbosch; C. E. J. de Ronde; B. T. Paul; J. B. Gemmell (811-824).
We analysed primary chalcopyrite from modern seafloor ‘black smoker’ chimneys to investigate high-temperature hydrothermal Cu isotope fractionation unaffected by metamorphism. Samples came from nine chimneys collected from Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau backarc basin. This is the first known study of Cu isotopes from submarine intraoceanic arc/backarc volcanoes, with both volcanoes discharging significant amounts of magmatic volatiles. Our results (n = 22) range from δ65Cu = −0.03 to 1.44 ± 0.18 ‰ (2 sd), with the majority of samples between ∼0.00 and 0.50 ‰. We interpret this cluster (n = 17) of lower δ65Cu values as representing a mantle source for the chimney Cu, in agreement with δ65Cu values for mantle rocks. The few higher δ65Cu values (>0.90 ‰) occur (1) within the same chimneys as lower values, (2) randomly distributed within the chimneys (i.e. near the top and bottom, interior and exterior), and (3) within chalcopyrite of approximately the same age (<1 year). This suggests the higher δ65Cu values are not related to oxidation by mixing with ambient seawater, but to isotopic variation within the vent fluids over a relatively short time. Theoretical studies demonstrate significant isotopic fractionation can occur between aqueous and vapourous complexing species. When combined with evidence for periodic release of magmatic volatiles at Brothers, we believe vapour transport of Cu is responsible for the observed isotopic fractionation. When compared to global δ65Cu data for primary chalcopyrite, volcanic arc chimneys are most similar to porphyry copper deposits that also form from magmatic-hydrothermal processes in convergent tectonic settings.
Keywords: Copper isotopes; Submarine hydrothermal systems; Massive sulphide deposits; Intraoceanic arc; Brothers volcano; Niuatahi volcano
Formation of the enigmatic Matoush uranium deposit in the Paleoprotozoic Otish Basin, Quebec, Canada by Paul Alexandre; Kurt Kyser; Daniel Layton-Matthews; Steve R. Beyer; Eric E. Hiatt; Jonathan Lafontaine (825-845).
The Matoush uranium deposit is situated in the Paleoproterozoic Otish Basin, northern Quebec, Canada, and is hosted by the Indicator Formation sandstones. Its sheet-like ore bodies are closely associated with the steeply dipping Matoush Fracture, which hosts mafic dykes and minor quartz–feldspar–tourmaline pegmatites. Regional diagenesis, involving oxidizing basinal fluids (δ2H ∼−15‰, δ18O ∼8‰), produced mostly illite and possibly leached U from accessory phases in the Indicator Formation sandstones. The bimodal Matoush dyke intruded the Indicator Formation along the Matoush Fracture, and the related metasomatism produced Cr-rich dravite and muscovite in both the dyke and the proximal sandstones. Uraninite formed when U6+ in the basinal brine was reduced to U4+ in contact with the mafic dyke and by Fe2+ in Cr–dravite and Cr–muscovite, and precipitated together with eskolaite and hematite. Because of its unique characteristics, the Matoush deposit cannot be easily classified within the generally accepted classification of uranium deposits. Two of its main characteristics (unusual reduction mechanism, structural control) do not correspond to the sandstone-hosted group of deposits (unconformity type, tabular, roll front), in spite of uranium being derived from the Otish Group sandstones.
Alteration, oxygen isotope, and fluid inclusion study of the Meishan iron oxide–apatite deposit, SE China by Jinjie Yu; Linrui Che; Tiezhu Wang (847-869).
The Meishan deposit (338 Mt at 39 % Fe) comprises massive ores in the main orebody and stockwork and disseminated ores along the main orebody. Four stages of mineralization and related alteration have been identified. The second stage of mineralization, which was the main stage of iron mineralization, formed stringer, disseminated iron ores, as well as the main Meishan orebody. The fourth stage formed small pyrite and/or gold orebodies above or alongside the main magnetite orebody. Stage 2 apatites have homogenization temperatures of 257–485 °C and salinities of 7.3–11 wt% NaCleq. Calculated δ18Ofluid values of magnetite and apatite from the disseminated ores vary between 7.7 and 14.9 ‰, which is similar to values observed in the massive ores (8.1–12.9 ‰). The high-18O fluids at Meishan have been interpreted as being of magmatic–hydrothermal origin. These fluids are indicative of the boiling of ore-forming fluids. Quartz, occurring as cavity fillings, gives homogenization temperatures from 202 to 344 °C, with most values lying between 250 and 330 °C. Corresponding salinities are ∼5 wt% NaCleq. Calculated δ18Ofluid values are +6.4 to +6.8 ‰. These values indicate that the lower-temperature (250–330 °C) quartz was deposited from a cooling magmatic–hydrothermal fluid. Stage 3 siderites contain fluid inclusions that homogenized between 190 and 310 °C, mainly between 210 and 290 °C. Corresponding salinities are 4–8 wt% NaCleq. Stage 4 quartz–carbonate veinlets contain fluid inclusions that homogenized at moderate to low temperatures (150–230 °C) and exhibit low salinities (2–10 wt% NaCl eq). δ18Ofluid values of the mineralizing fluids for the quartz and calcite can be calculated to vary from −0.7 to +5.6 ‰ and +6.3 to +10.2 ‰, respectively. While there is some overlap, the δ18O values of the fluids are generally lower than those observed in the massive and disseminated magnetite ores. δD values for the quartz and calcite vary between −154 and −123 ‰ and −123 and −111 ‰, respectively. These values suggest late-stage input of a shallow-sourced, isotopically light meteoric fluid at the temperature of pyrite and gold deposition. The fluid inclusion and stable isotope data indicate a cooling magmatic–hydrothermal system that progressed from isotopically heavy to isotopically depleted fluids as it cooled. Such fluid evolution is comparable with those of other Kiruna-type deposits worldwide.
Keywords: Iron oxide–apatite deposit; Porphyritic gabbro–diorite; Alteration zoning; Hydrothermal origin; Ningwu volcanic basin
In situ Sr isotope analysis of apatite by LA-MC-ICPMS: constraints on the evolution of ore fluids of the Yinachang Fe-Cu-REE deposit, Southwest China by Xin-Fu Zhao; Mei-Fu Zhou; Jian-Feng Gao; Xiao-Chun Li; Jian-Wei Li (871-884).
Apatite is a ubiquitous accessory mineral in a variety of rocks and hydrothermal ores. Strontium isotopes of apatite are well known to retain petrogenetic information and have been widely used to investigate the origin of igneous rocks, but such attempts have rarely been made to constrain ore-forming processes of hydrothermal systems. We here report in situ LA-MC-ICPMS Sr isotope data of apatite from the ~1660-Ma Yinachang Fe-Cu-REE deposit, Southwest China. The formation of this deposit was coeval to the emplacement of regionally distributed doleritic intrusions within a continental-rift setting. The deposit has a paragenetic sequence consisting of sodic alteration (stage I), magnetite mineralization (stage II), Cu sulfide and REE mineralization (stage III), and final barren calcite veining (stage IV). The stage II and III assemblages contain abundant apatite, allowing to investigate the temporal evolution of the Sr isotopic composition of the ore fluids. Apatite of stage II (Apt II) is associated with fluorite, magnetite, and siderite, whereas apatite from stage III (Apt III) occurs intimately intergrown with ankerite and Cu sulfides. Apt II has 87Sr/86Sr ratios varying from 0.70377 to 0.71074, broadly compatible with the coeval doleritic intrusions (0.70592 to 0.70692), indicating that ore-forming fluids responsible for stage II magnetite mineralization were largely equilibrated with mantle-derived mafic rocks. In contrast, Apt III has distinctly higher 87Sr/86Sr ratios from 0.71021 to 0.72114, which are interpreted to reflect external radiogenic Sr, likely derived from the Paleoproterozoic strata. Some Apt III crystals have undergone extensive metasomatism indicated by abundant monazite inclusions. The metasomatized apatite has much higher 87Sr/86Sr ratios up to 0.73721, which is consistent with bulk-rock Rb-Sr isotope analyses of Cu ores with 87Sr/86Sri from 0.71906 to 0.74632. The elevated 87Sr/86Sr values of metasomatized apatite and bulk Cu ores indicate that later fluids were dominated by highly radiogenic Sr equilibrated with the Paleoproterozoic country rocks. Results of this study highlight the utilization of in situ Sr isotope analysis of apatite in unraveling the evolution of hydrothermal systems.
Keywords: Apatite; In situ Sr isotope analysis; LA-MC-ICPMS analysis; Yinachang Fe-Cu-REE deposit; China
Discussion: The timing of gold mineralization across the eastern Yilgarn Craton using U–Pb geochronology of hydrothermal phosphate minerals by Roger Bateman; Sarah Jones (885-888).
The presentation of recent geochronological work on orogenic gold deposits in the Eastern Goldfields of the Yilgarn Craton, Western Australia, claims to prove that there is a single broad event of gold mineralization and that structural work demonstrating that there are a number of discrete gold mineralization events is wrong. This new data demonstrates no such thing, as this data, no doubt the best that can currently be produced, shows a very wide and inconsistent range in ages. Geochronology is not yet able to reliably separate these events, which appear to be spread over an interval of perhaps 30 Ma, up to ∼2635 Ma.
Keywords: Geochronology; Orogenic gold; Yilgarn; Kalgoorlie; Leonora
Reply to Discussion: The timing of gold mineralization across the eastern Yilgarn Craton using U–Pb geochronology of hydrothermal phosphate minerals by Noreen M. Vielreicher; David I. Groves; Neal J. McNaughton (889-894).