Fluids in distal Zn-Pb-Ag skarns: Evidence from El Mochito, Honduras
Zn-Pb-Ag mantos, chimneys, and skarns represent a spectrum of carbonate-hosted sulfide mineral deposits that have been collectively termed carbonate replacement deposits. Most such deposits cannot be related to plutons and, particularly for distal skarns, the role of magmatic versus other fluids (basinal brines and meteoric waters) has been uncertain. The El Mochito Zn-Pb-Ag deposit, Honduras, is an example of a large distal skarn, and comprises mantos and chimneys hosted mainly by limestones of the Early Cretaceous Tepemechin Formation. Previous isotopic studies indicate a magmatic source for the S and Pb and involvement of both magmatic and meteoric fluids in ore formation. The ore is hosted by garnet, magnetite, and pyroxene skarns, which developed sequentially from grandite- to andradite- to magnetite- and hedenbergite- rich skarns. Sphalerite and argentiferous galena occur interstitially to, or replace, the skarn minerals, with Fe- poor sphalerite (S1) principally associated with garnet skarn and Fe-rich (S2) sphalerite associated with pyroxene. Data from primary fluid inclusions show that the salinity of the mineralizing fluids decreased from ∼10-18 wt. % during the formation of garnet skarn and S1 sphalerite to ∼2-13 wt. % during pyroxene skarn formation and S2 sphalerite precipitation. Early, high-salinity fluids (∼33 wt. %) are represented by rare halite-saturated inclusions in garnet. Temperature increased from ∼ 365°C to ∼ 365°C from garnet/S1 sphalerite to pyroxene/S2 sphalerite, assuming a pressure of 500 bars. GC analyses indicate that the total concentrations of COv(2), CH4, and N2 were < 1 mole %. LA-ICPMS analyses were conducted mainly on inclusions in grandite and S1 sphalerite. The principal dissolved elements in the inclusions are Ca and Na, followed by K and Mn. The ore metals, Zn, Pb, and Ag, are present in high concentrations, with median values of 6000, 900, and 50 ppm, respectively. Element concentrations in fluid inclusions hosted by grandite have the same general ranges as those hosted by S1 sphalerite, except for Ca, which has a higher concentration in the grandite-hosted inclusions, consistent with the origin of grandite as a product of the replacement of limestone. The high temperatures require that magmatic fluids were important in skarn formation and ore mineralization. Variations in chemistry and temperature could be due to mixing of magmatic fluids with meteoric waters or basinal brines, however, the concentrations of K, Mn, Rb, As, Sb, Ba, Zn and Pb are very similar to those of inclusions of magmatic fluids trapped in granite-related mineral deposits and associated barren intrusions, as are the ratios of these elements to Na. The concentration of Ca is much higher in the El Mochito fluids, as might be expected for a deposit that formed by replacement of limestone. In contrast, the El Mochito fluids are distinctly different from basinal brines, and the data indicate no mixing between magmatic fluids and basinal brines. Rather, variations in salinity and elemental chemistry are interpreted to have resulted from changes at source. The Pb/Zn ratio of the El Mochito fluids ranges from 0.1 to 1.7, which encompasses the average ratio for the deposit (0.29). This suggests that Zn and Pb were precipitated in proportions that approach their ratios in the fluid.
Stratigraphic controls of calc-silicate alteration and Cu-Au mineralization in the Deep MLZ skarn, Ertsberg District, Papua, Indonesia
The Ertsberg District includes diverse copper-gold orebodies associated with Pliocene plutons that were emplaced into deformed Mesozoic and Cenozoic sedimentary sequences that form the Central Ranges of New Guinea. The Deep Mill Level Zone (DMLZ) skarn Cu-Au deposit is the deepest explored part of the giant Ertsberg East Skarn System that extends from the surface at 4200 m elevation to the DMLZ at 2900 to 2600 m. Current MLZ/DMLZ undeveloped reserves are 400 million tonnes at 1.0% Cu and 0.8 g/t Au. The DMLZ is hosted in an Upper Cretaceous to Lower Tertiary siliciclastic and carbonate succession adjacent to the 3-Ma Ertsberg pluton. Much of the DMLZ ore is hosted by mixed assemblages of siliciclastic and dolomitized carbonate strata that have been altered to Mg-rich skarn assemblages. Unaltered equivalent strata from a 300- m cored interval in the Yellow Valley Syncline were characterized petrographically, and representative samples were analyzed for major element compositions. These studies confirm the compositional variability of the Waripi sequence that can be principally assigned to the relative percentage of quartz (mostly detrital, with minor microcrystalline silica), calcite, and dolomite. These stratigraphic and chemical units are interpreted to be responsible for the varying skarn types within the DMLZ. The dominant prograde skarn assemblages are controlled by protolith composition, notably the relative abundance of quartz, dolomite, and calcite within the protolith. Models based on isochemical metamorphism of mixed assemblages of quartz and dolomite suggest that the formation of the forsterite-diopside dominant skarn assemblages resulted in the greatest amount of pore space increase. The location of DMLZ Cu-Au concentrations was controlled at a local scale by host lithology, with structural features providing pathways that focused the mineralizing fluids.
Proterozoic Carbonate Lithofacies Control the Distribution of Sulphides at the Gayna River Zn-Pb Camp, Mackenzie Mountains, NWT
Zn-Pb deposits at Gayna River, NWT are predominantly concentrated in the informal 'Grainstone formation', a dolostone of the early Neoproterozoic Little Dal Group (Mackenzie Mountains Supergroup). Previous work showed that the mineralisation (inferred 50 Mt combined from numerous zones; 5 percent combined Zn+Pb) is fracture-controlled and spatially associated with giant stromatolite reefs (500 m thick) of the underlying formation. The rheologically brittle, uncompactable and hydrologically tight reef masses are enclosed by coeval, compacted shale and deep-water limestone. A long and complex history of reef growth controlled by sea-level change resulted in a distinctive reef morphology that includes a sharp right-angle at all reef-top margins, where heterogeneous, off-reef limestone, shale and dolostone abut the rigid, lithologically homogeneous reefs. These zones of abrupt lateral facies change, between uncompactable reef and ductile, layered off-reef strata, represent the structurally weakest points in the system, where, during even subtle later tectonic events, stress would be preferentially accommodated. Brittle deformation of competent carbonate layers in this inflection zone in response to stress produced fracture haloes around reef-tops, which were then occluded by Zn-Pb sulphides. Abrupt competence contrasts appear to be necessary for the production of fractures that control the locations of sulphides at Gayna River. The dominant fractures in the Gayna River camp are those associated with reef- tops. The plan shape and location of buried reef-tops are probably the most critical controls on the distribution of hitherto undiscovered sulphide masses in the subsurface. Careful mapping of those depositional lithofacies that are characteristic of near-reef environments and of subtle, compaction-related dips in appropriate stratigraphic levels may provide vectors to as-yet unrecognised subsurface reef-margin zones favourable for Zn-Pb mineralisation. Structures and lithofacies presently exposed at surface indicate the almost certain existence of buried reefs in the immediate vicinity of some of the known showings.
The genesis of the Gayna River carbonate-hosted Zn-Pb deposit, Northwest Territories, Canada
The Gayna River deposit is located in the Mackenzie Mountains and is hosted by the platform carbonates of the Neoproterozoic Little Dal Group. The mineralization occurs as a series of sphalerite cemented breccias and veins and has the following main paragenetic components: pre-ore (pyrite, limestone, dolostone, calcite cement and early sphalerite), ore-related (dolospar, quartz, sphalerite, and galena) and post-ore (quartz, pyrite, calcite, and pyrobitumen). The pre-ore phase pyrite and sphalerite samples have low ä34S values (-4.4 to 2.4‰ and 1.5 to 5.2‰ respectively). Carbon and oxygen isotope results for the host dolomite were ä13C = 1.8 to 4.6‰ (PDB) and ä18O = 22.1 to 25.2‰ (SMOW). The ore-phase sulphides have ä34S values that range from 12.2 to 25.7‰. The dolospar has ä13C values of 0.7 to 2.5‰ (PDB) and ä18O values of 20.0 to 26.6‰ (SMOW). Primary fluid inclusion data in sphalerite suggest the main phase of mineralization was formed by fluids that ranged in homogenization temperature (Th) from 107 to 227°C and 23.0 to 34.4 wt.% NaCl equiv. Pyrites from the post-ore phase have ä34S values of 11.5 to 21.2‰. The late calcite phase had ä13C values of -3.0 to 3.0‰ (PDB) and ä18O values of 16.6 to 20.3‰ (SMOW). Inclusions in late quartz and calcite have Th values that range from 99 to 233°C and 59 to 159°C respectively and corresponding salinities of 13.7 to 32.9 wt % NaCl equiv. and 13.6 to 23.7 % NaCl equiv for calcite. Re-Os dating of the pyrobitumen suggests a Tertiary age for this parageneic stage These data suggest that the Gayna River deposit likely was formed by mixing of at least two fluids and one of those fluids originated as seawater. The sulphur for the deposit was derived from seawater sulphate by thermochemical sulphate reduction. Post-mineralization fluids are cooler and less saline, possibly due to the mixing of the mineralizing brine with a more dilute fluid.
The Mixing Hypothesis and the Origin of MVT Deposits