The Geology and Geochemistry of Base Metal Sulfide Mineralization in the Foster River Area, Northern Saskatchewan: A SEDEX Deposit With Broken Hill-Type Affinities
The Foster River area, northern Saskatchewan, is one of several Pb-Zn and Zn deposits (e.g. George Lake: 7.8 Mt @ 3.9% Zn and 0.5% Pb) along the SE margin of the highly deformed and metamorphosed Wollaston Domain. Sulfide mineralization in the Foster River area (2.00-1.85 Ga) occurs near the middle of the Wollaston Supergroup in quartzites within a unit of psammites and pelites that were metamorphosed to the upper amphibolite facies and subjected to at least four episodes of deformation. Drilling indicates that the Sito Lake East prospect contains 50,000 t of 4.5% Zn, with one intercept containing 11m of 4.2% Zn and 0.6% Pb. A boulder from a boulder train contains up to 13.2% Zn, 4.0% Pb, and 11 g/t Ag. Sulfides are spatially associated with a package of rocks similar to that spatially associated with Broken Hill-type (BHT) deposits (quartzite, gahnite-rich rocks, iron formation, and quartz garnetite). A nodular sillimanite rock that occurs in the vicinity of the Sito Lake East prospect is likely to be part of a stratabound hydrothermal alteration zone. Hydrothermal mineralization consists of pyrite, sphalerite, pyrrhotite, galena, chalcopyrite, magnetite, and graphite whereas gangue minerals include garnet, gahnite, tourmaline, calcite, and augite. Chondrite-normalized REE patterns of silicate-facies iron formation (garnet-pyroxene-amphibole-magnetite rock) and quartz-garnetite show light REE enrichment and heavy REE depletion with moderate to large negative Eu anomalies. Such anomalies are consistent with meta-exhalites that have a high detrital component (>30%) in the source rock. The compositions of garnet (spessartine-almandine) and gahnite at Foster River are similar to those spatially associated with BHT deposits. Sulfur isotope compositions of pyrite, pyrrhotite, and sphalerite from the Foster River area range from 26.2-38.1 per mil (n=20) and are consistent with sulfur being sourced from Proterozoic seawater that was modified by microbial sulfate reduction in a restricted basin. The low Pb/(Pb+Zn) ratio of sulfide mineralization and the lack of bimodal volcanics suggest that the mineralization in the Foster River area is a SEDEX deposit with BHT affinities.
Classification of Broken Hill-Type Pb-Zn-Ag Deposits: A Refinement
Broken Hill Hill-type Pb-Zn-Ag (BHT) deposits constitute some of the largest ore deposits in the world. The Broken Hill deposit is the largest accumulation of Pb, Zn, and Ag on Earth and the Cannington deposit is currently the largest silver deposit. Characteristic features of BHT deposits include: 1. high Pb+Zn+Ag values with Pb > Zn; 2. Metamorphism to amphibolite-granulite facies; 3. Paleo-to Mesoprotoerozoic clastic metasedimentary host rocks; 4. Sulfides that are spatially associated with bimodal (felsic and mafic) volcanic rocks, and stratabound gahnite- and garnet-bearing rocks and iron formations, 5. Stacked orebodies with characteristic Pb:Zn:Ag ratios and skarn-like Fe-Mn-Ca-F gangue assemblages, and the presence of Cu, Au, Bi, As, and Sb; and 6. Sulfur-poor assemblages. Broken Hill (Australia) has a prominent footwall feeder zone whereas other BHT deposits have less obvious alteration zones (footwall garnet spotting and stratabound alteration haloes). Deposits previously regarded in the literature as BHT deposits are Broken Hill, Cannington, Oonagalabie, Menninie Dam, and Pegmont (Australia), Broken Hill, Swartberg, Big Syncline, and Gamsberg (South Africa), Zinkgruvan (Sweden), Sullivan, Cottonbelt, and Foster River (Canada), and Boquira (Brazil). Of these deposits, only the Broken Hill (Australia, South Africa), Pinnacles, Cannington, Pegmont, and Swartberg deposits are BHT deposits. Another BHT deposit includes the Green Parrot deposit, Jervois Ranges (Northern Territory). The Foster River, Gamsberg, and Sullivan deposits are considered to be "SEDEX deposits with BHT affinities", and the Oonagalabie, Green Mountain (Colorado), and Zinkgruvan are "VMS deposits with BHT affinities". In the Broken Hill area (Australia), Corruga-type Pb-Zn-Ag deposits occur in calc-silicate rocks and possess some BHT characteristics; the Big Syncline, Cottonbelt, Menninie Dam, and Saxberget deposits are Corruga-type deposits. SEDEX deposits with BHT affinities, VMS deposits with BHT affinities, and Corruga-type deposits represent transitional deposits between BHT and SEDEX, VMS, and metamorphosed base metal calc-silicate deposits, respectively. Although the non-sulfide zinc deposits at Franklin Furnace and Sterling Hill, NJ, do not contain Pb, they resemble sulfur-poor BHT deposits.
Silica Transport and Distribution in Saline, Immiscible Fluids: Application to Subseafloor Hydrothermal Systems
Quartz is a nearly ubiquitous gangue mineral in hydrothermal mineral deposits, most often constituting the bulk of hydrothermal mineralization. The dissolution, transport and precipitation of quartz is controlled by the solubility of silica; in particular, in hot hydrothermal fluids in contact with quartz, silica saturation can generally be assumed, as rates of dissolution and precipitation are generally much faster than fluid flow rates. The solubility of silica in aqueous fluids can be used to understand the evolution of hydrothermal systems by tracing the silica distribution in these systems through time. The solubility of quartz in an aqueous fluid is dependent upon the pressure, temperature and composition (PTX) of the fluid. Silica solubility in pure water as a function of pressure and temperature is well understood. However, natural fluids contain variable amounts of dissolved ionic species, thus it is necessary to include the effects of salinity on silica solubility to accurately predict quartz distribution in hydrothermal systems. In particular, addition of NaCl results in enhanced quartz solubility over a wide range of PT conditions. Furthermore, if phase separation occurs in saline fluids, silica is preferentially partitioned into the higher salinity brine phase; if vapor is removed from the system, the bulk salinity in the system evolves towards the brine end member, and overall silica solubility is enhanced. There is abundant evidence from natural fluid inclusions for fluid immiscibility in hydrothermal ore deposits. Additionally, recent hydrothermal models that include fluid phase equilibria effects predict that phase separation may be an important control on the distribution of dissolved components in seafloor hydrothermal systems. An empirical equation describing the solubility of silica in salt-bearing hydrothermal solutions over a wide range of PTX conditions has been incorporated into a multiphase fluid flow model for seafloor hydrothermal systems to predict the evolution of silica distribution in time and space in these systems. Preliminary runs illustrate a significant effect of salinity in the evolving and boiling system on silica solubility. The model predicts that silica solubility is progressively enhanced in the two-phase liquid-plus-vapor region as brine is concentrated by the preferential loss of vapor. The model also predicts that there is a narrow region of intense quartz deposition in the deep part of the upflow zone, where the fluid reenters the one-phase field. The model currently treats the wallrock as an infinite quartz reservoir, but future work will fully couple the quartz solubility and fluid flow models, to allow porosity adjustment and resultant permeability evolution by quartz dissolution and precipitation. This work was supported in part by the Institute for Critical Technology and Applied Sciences (ICTAS) at Virginia Tech
Measurements of Lightning During the Grimsvotn 2004 Eruption in Iceland
Lightning activity in volcanic ash plumes is common, especially in subglacial or submarine eruptions. The interaction between magma and water may be responsible for electric charge separation, leading to positively charged vapor and negatively charged ash. Lightning data were collected during the last three volcanic eruptions in Iceland; Grimsvotn 1998, Hekla 2000 and Grimsvotn 2004. For the last eruption we collected data from the LLP Icelandic lightning location system, the ATD sferics system of the UK Met Office, as well as from our vertical E-field wave recording station, located in Reykjavik. We note a good correlation between the lightning activity and the intensity of the eruptions as indicated by the height of the ash plume observed by weather radar. The lightning data collected during these volcanic eruptions gives valuable insight into the character of volcanogenic lightning and how they differ from weather lightning.
Volatile Evolution of Magma Associated with the Solchiaro Eruption in the Phlegrean Volcanic District (Italy)
The Phlegrean volcanic district (PVD) in southern Italy is one of the best known volcanic hazard areas in the world. More than 1.5 million people live in close proximity to the volcanic centers. The PVD includes the Campi Flegrei caldera and the islands of Ischia and Procida. We studied volatile and major element compositions of magma associated with the Solchiaro eruption on the Island of Procida, Italy, to gain a better understanding of the relationship between pre-eruptive volatiles and magmatic evolution. The Solchiaro eruption is one of the more primitive products erupted in the PVD and provides information on the source of the later more evolved magmas associated with this volcanic system. The composition of the magma before eruption was determined by analyzing melt inclusions (MI) in forsteritic olivine and diopside, glass embayments in phenocrysts, rim glass on phenocrysts and highly vesiculated matrix glass. The composition of MI ranges from basaltic to trachy-basaltic. Among major elements potassium shows the highest variability, ranging from 0.86 to 4.24 wt %. Three types of melt have been identified based on chemistry and occurrence: type 1 related to MI with Mg# > 66, type 2 represented by MI with Mg# < 66 and type 3 representing glass phases with Mg# intermediate to the other two. Type 1 melt is volatile undersaturated and was trapped as MI at relatively high depth (crust-mantle boundary) and is linked to source processes. Type 2 melt is the result of fractional crystallization and assimilation (FCA) of type 1 melt and was saturated in volatiles. Pressures of crystallization range from 0.62 to 1.43 kbar, or depths of 2.2 to 5.3 km. Type 3 melt represents the melt at the moment of the eruption. H2O-CO2 analyses of this glass shows the best fit to a calculated degassing path. The reconstructed magmatic evolution suggests that only in type 2 melts is there a positive correlation between Mg# and pressure or CO2 content. Compositions of some MI hosted in Mg-rich olivine show extremely low K2O and Na2O and high CaO contents of 0.86, 1.90 and 13.55 wt.%, respectively. Such compositions have not been previously reported for the PVD but are characteristic of the Aeolian Arc to the southwest.
Geochemistry, Metamorphism, and Partial Melting of Hydrothermally Altered Rocks in the Sherridon Complex, Trans Hudson Orogen, Manitoba
Lithogeochemistry and field investigations of regionally metamorphosed rocks in the central portion of the Sherridon Complex of the Trans-Hudson orogen indicates high-grade gneisses are dominantly derived from protoliths with rhyodacite and basaltic compositions that subsequently experienced pre-metamorphic seafloor- related hydrothermal alteration. Immobile trace element geochemistry results indicate the voluminous rhyodacite composition rocks formed in a volcanic arc environment, and associated VMS deposits and less voluminous basaltic composition rocks suggest a bimodal-felsic VMS environment. Felsic lithologies are interpreted to have experienced local pre-metamorphic sericite, chlorite, carbonate, and possible silica alteration, with an extensive zone of carbonate alteration associated with more basaltic-composition protoliths. Upper amphibolite facies metamorphism of hydrothermally altered felsic lithologies resulted in a variety of quartz-rich rocks containing combinations of gahnite, garnet, cordierite, sillimanite, orthoamphibole, biotite, and feldspar bearing assemblages. An extensive garnet-cordierite-sillimanite-biotite ± orthoamphibole quartz-rich gneiss unit (GCSB) grades into a stromatic migmatite with a modal decrease in garnet and sillimanite in the host gneiss. Cordierite commonly overgrows and locally completely replaces sillimanite. Increased amounts of melting results in very local diatexite and locally mobilized melt crystallizing to a massive biotite clotted granitoid. The biotite-rich clots in the granitoid are interpreted to have resulted from replacement of garnet ± cordierite within the melt based on their shape, small remnants of garnet, and local sillimanite. Phase equilibria modelling of a suite of GCSB rocks (average Mg# = 0.55) in the MnNCKFMASHT chemical system utilizing program Domino predicts pressures in excess of 5.5 kilobars for sillimanite stability, and is most compatible with pressures between 6.5-7.0 kilobars based on sillimanite and cordierite abundance in samples without evidence of melting. Assuming fluid-excess subsolidus conditions, temperatures are predicted to have reached at least 685° C to achieve melting in the compositions used for modelling. Cordierite overgrowths on sillimanite and garnet consumption are most compatible with a component of decompression at peak or near-peak temperatures. Maximum temperatures are predicted to be less than 710° C if decompression led to pressures less than 4 kilobars during melting based on the absence of observed orthopyroxene. This portion of the Trans Hudson orogen is therefore interpreted to have experienced a clockwise regional metamorphic P-T path with a component of high-temperature decompression.
Magma Injection as a Trigger for Dome Collapse Eruption of the Soufriere Hills Volcano, Montserrat: Evidence from a Single Pyroclast
A single, pumiceous pyroclast collected following a major (85 million m3) dome collapse eruption of the Soufriere Hills volcano, Montserrat (21 September 1997) was found to contain textural and geochemical evidence for: 1) multiple decompression events, and 2) magma injection into the dome at the onset of dome collapse. Quartz, plagioclase and hornblende phenocrysts contain a variety of closed and burst melt inclusions (MIs). Burst MIs occur on fracture surfaces within exploded phenocrysts, with melt extruded as either foam or cylindrical pipe-forms. Fractures within exploded quartz phenocrysts are infilled with glass, whereas those within plagioclase are entirely free of glass, indicating relatively early fracturing (decompression) of the quartz. This interpretation is supported by slightly higher measured H2OT in burst MIs in quartz (av. 0.45 wt%) compared with plagioclase (av. 0.34 wt%), indicating equilibration on bursting of the quartz at higher relative pressures. From the low levels of H2OT in glass from the burst MIs in quartz, this fracturing occurred within the dome or upper conduit/dense plug at pressures < 10 MPa, probably at a depth much less than 400 m. The fracturing of plagioclase occurred at even higher levels. Both the fracturing of quartz and plagioclase occurred whilst the matrix was still molten, as evident from the presence of stretched vesicles in matrix glass at fracture terminations. The fractures in plagioclase are empty of vapour phase precipitates (such as cristobalite), whereas the pumiceous vesicles commonly contain such crystals, and therefore the fracturing of plagioclase is thought to have been the last depressurisation event recorded in the pyroclast. Indeed the fracturing is considered to have occurred due to the overpressure caused by the dome collapse which resulted in the pyroclast being ejected from the dome. We suggest that this is the first direct evidence for the injection of magma into the dome at the onset of dome collapse, and that this may have been the trigger for dome collapse itself.
Unusual Volcanic Products From the 2008 Eruption at Volcan Llaima, Chile
Volcan Llaima, a snow-covered basaltic andesite stratocone in southern Chile (38º 41' S, 71º 44' W, 3179 m a.s.l.), erupted on 1 January 2008 with a fire fountain display lasting 14 hours. Elevated activity continues to date with mild to moderate strombolian activity occurring from two nested scoria cones in the summit crater and with occasional lava flows from crater overflow. The eruption displayed contrasting styles of activity emanating from different parts of the edifice that may provide some unique insight into the upper level plumbing system. Furthermore, the activity has provided an excellent chance to study the transition of a normally passive degassing system into a violent eruptive cycle. A field study of the eruptive products from this eruption was completed in January 2009, where sampling was carried out from the tephra fall, lava flows, lahar deposits and even small pyroclastic flow deposits. The scoria samples collected suggest a mixture of two magmas involved in the initial violent, fire fountaining activity from the summit. Additionally, they exhibit a variety of unusual textures, including rapidly-quenched, dense lava 'balls' - generated at the front of the lava flows traveling through ice, as well as cauliflower-textured tephra from explosive eruptions though ice. This presentation comprises our observations and preliminary interpretations concerning the processes that occurred during this unique eruption.