Paleomagnetic Age for the World-class Century Zn-Pb-Ag Deposit, Australia
Paleomagnetic results are reported for the Century Zn-Pb-Ag SEDEX deposit in northwestern Queensland, Australia. The stratiform mineralization occurs in fine parallel lamellae in ∼1595 Ma siderite-rich siltstones and black shales of the upper Lawn Hill Formation in the Proterozoic McNamara Group. Galena from the deposit has given a Pb/Pb model age of ∼1575 Ma. Paleomagnetic analysis of 333 specimens from ore zones (15 sites), and hanging wall (4 sites) and footwall (5 sites) siltstones using mostly thermal and then alternating field step demagnetization, isolates a stable characteristic remanent magnetization (ChRM) for the ore sites only. Step demagnetization, rock magnetic tests and thermomagnetic analyses of ore, Zn and Pb concentrates and tailings show that the main remanence carriers are single- or pseudosingle-domain inclusions of titanomagnetite in sphalerite and gangue, and pyrrhotite in galena with modern goethite and/or hematite from the weathering of siderite. A paleomagnetic fold test using the ore sites is positive, showing that the ore ChRM predates D2 deformation in the ∼1595 to ∼1500 Ma Isan orogeny. The orogeny folded the main-stage mineralization, indicating that the ore retains a primary magnetization. The optimum 80% tilt-corrected unit mean ChRM direction for the ore gives a Mesoproterozoic paleopole at ∼1560 Ma on the northern Australian apparent polar wander path. Thus this result both constrains the timing of mineralization and provides an upper age limit for D2 deformation in the orogeny.
Tectonic Deformation-Driven Fluid Flow Associated With the Formation of the Chanziping Uranium Deposit, Guangxi, China: theoretical results from computational modeling
Abstract The Chanziping uranium deposit straddles the Guangxi and Hunan provinces of southern China, and is structurally-hosted below the unconformity between the Mesozoic red basin fill and metamorphic basement. It is thought that the deposit may form by the mixing of an oxidizing fluid circulating in basinal sandstones with a reducing fluid resulting from basement rocks at or near the unconformity. However, it is unclear what actually controlled the focusing of ore-forming fluids into the specific areas. This paper represents the first numerical investigation of fluid flow associated with the formation of the Chanziping uranium deposit. A 2-D conceptual model was developed to fully coupled tectonic deformation and fluid flow using a finite difference code FLAC. A series of numerical case studies were conducted to quantify the role of structural control in governing fluid flow and the resultant ore-forming processes. Geological implications of our numerical results were also discussed.
Finite element modeling of hydrothermal fluid flow in Peace River Arch of Western Canada Sedimentary Basin: implications for dolomitization
A finite element computer modeling approach, integrated with existing geological, geochemical and geophysical data, was used to address the diagenetic process of dolomitization in Western Canada Sedimentary Basin (WCSB). A 2-D conceptualized model was developed to simulate hydrothermal flow in particular for the packland play type dolomitization in Peace River Arch of WCSB. Our numerical results indicate that faults serve as important pathways for the ascending hydrothermal fluids driven by buoyancy force due to temporal and spatial changes in temperature. Both steady state and transient computations were conducted to reveal suitable hydraulic conditions under which the modeled temperature within the aquifer system is consistent with observed values in the targeted study area. A series of numerical case studies were carried out to investigate key factors controlling hydrothermal fluid flow, including fault penetration depth, width and permeability, and its connectivity with the host rock units.
Controls on Zn(-Pb) Mineralization in the Lower Cambrian Sekwi Formation, Mackenzie Mountains Zinc District, Northwest Territories
The Mackenzie Mountains Zn district includes over 200 Zn(-Pb) showings hosted by Neoproterozoic to Lower Paleozoic carbonate rocks. This study addresses three of over 30 known showings in the Lower Cambrian Sekwi Formation, over 130 km of strike length (AB, TIC and Palm showings, from NW to SE). Property-scale mapping and detailed stratigraphy of mineralized successions are correlated with composite sections assembled from existing data to determine the stratigraphic context of each showing. Mineralization is present in the upper part of the formation and its uppermost lower part. Host lithologies, including dolograinstone and quartz-silty, burrowed dolostone, have mm- to cm-scale variations in mineralogy and thus in alteration potential. Void-filling, carbonate-sulphide-cemented breccia volumetrically exceeds replacive mineralization. Brecciation and local dissolution of host rocks preceded mineralization, whereas dolomitization continued after mineralization was complete. Faulting is spatially associated with each showing, although its expression is subtle and its role unclear. Fluid inclusion thermometric data for AB and TIC indicate involvement of two fluids: one with Th of ∼150°C and one with Th of 200-235°C. Salinities range from 2 to 22 wt% equiv. NaCl. Fluids in the NW area are slightly less saline (2-18 wt% vs. 8-22 wt% in the SE). 87Sr/86Sr ratios of sphalerite and dolomite suggest interaction of mineralizing fluids with a radiogenic reservoir inferred to be basement. Values are similar for AB and Palm (0.712 to 0.720), but higher values of 0.721 to 0.730 indicate a difference in flow path or duration for TIC. δ34S values for sphalerite are similar at AB and TIC (+10.1 to +20.6 ‰). Sulphur was derived from one source, likely evaporitic sulphate, and was reduced by TSR. Sphalerite at Palm has markedly negative δ34S values (-2.6 and -6.0 ‰) consistent with biogenic reduction of the same evaporitic sulphate. Sulphide precipitation may have been initiated by mixing of cool brine with a warmer fluid that passed through basement and up via faults into already-buried host rocks.
Preliminary investigation of the nature and origin of the Sierra Mojada Non-sulfide Zn deposits, Coahuila, Mexico
The Sierra Mojada district consists of multiple types of mineral concentrations ranging from polymetallic sulfide deposits, 'non-sulfide Zn' deposits (separate smithsonite and hemimorphite zones), and a Pb carbonate manto hosted by Upper Jurassic to Lower Cretaceous carbonates. The district is typically grouped with other polymetallic carbonate replacement deposits of southwestern North America, but the intrusive rocks that commonly are associated with these types of deposits are not known at Sierra Mojada. The Sierra Mojada district located near the boundary of the Coahuila Platform and the Sabinas Basin that formed during Late Jurassic and Cretaceous tectonic extension. The east-trending San Marcos fault runs through the Sierra Mojada district and is thought to have acted as the major conduit for basinal fluids, responsible for local dolomitization, sulfide mineralization, and petroleum in the region. The polymetallic sulfides are the stratigraphically and structurally highest ore zone and occur north of the San Marcos fault. Non-sulfide Zn (hemimorphite, smithsonite and sauconite) and lead (cerussite) concentrations occur south of the fault in the Aurora and La Pena Formations. The Iron Oxide Manto consists of stratabound zones of dominantly of hemimorphite pore-filling in Fe-oxide rich dolostones giving the ore a distinct red to orange color. The Smithsonite Manto has distinct karst features, including internal sediments interbanded with smithsonite in the lower part of the orebody. The non-sulfide Zn ores are being studied to document their morphologies, growth patterns, and paragenetic relationships using conventional petrography, SEM, and CL-based ESEM. In the Smithsonite Manto, banded and colloform smithsonite consists of aggregates of rhombohedral crystals that grew into open space and also occurs within internal sediment bands with hemimorphite or Zn clays and Zn oxides. Black dendrites of Mn oxides with Fe oxides are encased in banded smithsonite. Euhedral hemimorphite is present in both mantos, although it is most abundant in the Iron Oxide Manto. Hemimorphite occurs in the Smithsonite Manto as layers with smithsonite suggesting that they may have precipitated together, although locally hemimorphite cross-cuts smithsonite bands. Locally, hemimorphite and smithsonite have been altered to sauconite and hydrozincite. Associated minerals include barite and calcite that seem to have formed later than the major Zn mineral formation. Preliminary isotope studies reveal that smithsonites from Sierra Mojada have δ18OVSMOW values ranging from 19.1 to 22.1 ‰ and δ13CVPDB values of -7.0 to +1.0 ‰. While the range of carbon isotope values is typical for supergene smithsonites, the oxygen isotope values are much lower than those recorded in most supergene deposits. The unusually low oxygen isotope values in smithsonite require either highly 18O- depleted waters of less than -12 ‰ if oxidation occurred at temperatures of less than 20C° or elevated temperatures (>40C°) during oxidation if waters had isotope compositions similar to present- day ground waters in the area. Studies are in progress to further constrain the paragenesis, mineralizing fluid character, and timing of mineralization in the Sierra Mojada district.
Chemical Variation of Inclusion Fluids in Ozark MVT Deposits Determined by LA-ICP-MS: Implications for Deposit Size and Ore Precipitation Mechanisms
The Ozark Plateau of the central U.S. is one of the most important provinces of Mississippi Valley-type (MVT) Pb-Zn mineralization in the world. The province includes the giant Southeast Missouri and Tri-State districts and the much smaller though still significant Northern Arkansas district. The three districts have long been regarded to be genetically related on the basis of their similar Pennsylvanian-Permian ages, fluid inclusion salinity and homogenization temperatures, and apparent association with a regional south-north groundwater flow event. However, differences in the size, mineralogy, and stratigraphic position of the districts raise the question of whether the districts differed with respect to ore fluid composition and precipitation mechanism, a possibility that is suggested by previously published fluid inclusion studies. In addition, because most of the Ozark Plateau was not significantly mineralized by the regional groundwater flow event, the question is raised of whether the ore fluids in the three districts were geochemically anomalous compared to typical sedimentary fluids. The present study has sought to investigate these questions through ongoing LA-ICP-MS analysis of fluid inclusions hosted in sphalerite, dolomite, and quartz. An important geochemical similarity among the three districts is that sphalerite-hosted fluid inclusions in each district have a Pb-rich population that is not present in the gangue-hosted fluid inclusions. This suggests that a metal-rich brine invaded the districts during the time of sulfide mineral deposition, possibly mixing with a resident fluid that induced mineralization. Compositional differences also exist among the districts. For example, some of the sphalerite-hosted fluid inclusions from Southeast Missouri have K-Na ratios that are much higher than in any fluid inclusions from the other two districts. The high K-Na ratios do not correlate with high aqueous Pb concentrations though the sphalerite that hosts the high K-Na fluids has high matrix Pb concentrations. Tri-State and Southeast Missouri exhibit bimodal salinities whereas Northern Arkansas is unimodal. The bimodal salinity of Southeast Missouri and Tri-State implies a greater salinity contrast between the invading and resident fluids, which perhaps played a role in deposit size. Inclusion fluids in sphalerite from Tri-State exhibit elevated Ca and Sr concentrations compared to Northern Arkansas whereas Southeast Missouri has both a Ca-Sr-enriched and depleted population of fluid inclusions relative to Northern Arkansas. In addition to being Pb-rich, some of the sphalerite hosted fluid inclusions from Tri-State and Northern Arkansas are also Cu-rich. Thus, although all three districts appear to have formed by mixing of an invading metal-rich brine with a resident groundwater, neither fluid was homogeneous in composition across the Ozark Plateau.