The Geohydrology of MVT-Ore Genesis in the Canning Basin, Western Australia
In the Lennard Shelf, Western Australia, epigenetic MVT-type Pb-Zn mineralization occurs in Middle Devonian evaporitic dolomites which were part of a barrier reef system (Hurley & Lohmann, 1989). Ore mineralization exhibits a strong structural control at the basin scale and normal faults probably controlled pathways for brine and petroleum migration that affected ore deposition (Wallace et al., 1999). For the Canning basin, finite element simulations show that compaction was the most important process for creating overpressures and driving basinal fluids in this thick extensional basin. Basinal fluids are shown to have been driven across the Fitzroy Trough through permeable and deeply buried Silurian-Ordovician aquifer units. The fluids then migrated upwards at rates of m/yr up during periods of episodic extension (Braun, 1992) where fluid flow was channeled by major normal fault zones like the Cadjebut and Pinnacles Faults. Reactive flow simulations test a petroleum-reservoir model for mineralization whereby metal-bearing brines mix with accumulated hydrocarbons (Anderson & Garven, 1987). The results show that compaction-driven flow, as proposed by Beales & Jackson (1966) and Jackson & Beales (1967), works rather well in this ore district--other mechanisms such as sealevel tidal pumping (Cathles, 1988) or topographic drive (Solomon & Groves, 1994) are more tenuous and really unnecessary from a mass transport or geohydrologic basis.
Paleomagnetic Constraints on the age of the Lisheen Zn-Pb Deposit, Ireland: A Pre- Variscan Metamorphosed "MVT" Versus an Epigenetic Variscan Model for Ore Genesis
Lower Carboniferous carbonate units in the Irish Midlands host major Zn-Pb ore deposits in two units, the Navan Group and the Waulsortian Limestone. The age and, therefore, the genesis of these ore deposits remains controversial because of the lack of absolute geochronological constraints. In addition, the effect of the Early Permian Variscan thermal episode, observed by elevated conodont color alteration indices in all Carboniferous strata in Ireland, on the Zn-Pb ore deposits is not clearly understood. This paleomagnetic study was undertaken to date and, thereby, constrain the genesis of the Waulsortian Limestone-hosted Lisheen Zn- Pb ore deposit. Specimens (432) from 12 sites in ore mineralization and 10 sites in host rocks at Lisheen were subjected to alternating-field and thermal step demagnetization protocols. Analysis of these specimens isolated a well-defined stable shallow and southerly-up paleomagnetic characteristic remanent magnetization (ChRM) direction. Saturation remanence tests, thermal decay data, and a paleomagnetic tilt test indicate a post-folding ChRM that is carried dominantly by single-domain magnetite. The ChRM directions from 8 host rock and 11 Zn-Pb mineralized sites are indistinguishable at 95% confidence, and give a mean paleopole at 41.6° S, 18.8° W (dp = 1.7°, dm = 3.3° ) with a paleomagnetic age of 277 ± 7 (2 σ) Ma on the apparent polar wander path for Laurentia in European coordinates. This Early Permian magnetization postdates peak-Variscan orogenic heating to ∼ 350° C in the surrounding region, suggesting two basic genetic models for Lisheen's Zn-Pb mineralization i.e. Variscan and metamorphosed pre-Variscan. The Variscan model, our preferred interpretation, suggests that the Zn-Pb mineralizing event occurred at 277 Ma during cooling from the regional Variscan thermal episode. This model, in conjunction with other thermal data, supports an entirely epigenetic origin that invokes a topographically-driven fluid flow, either from the south of Ireland or from closer to the deposit, that was modified by localized convection during the Variscan Orogeny. In contrast, the pre-Variscan model - favoured by previous authors - requires heating of the original Carboniferous Zn-Pb deposit to temperatures equivalent to chlorite-facies greenschist metamorphism during the Variscan Orogeny.
Dating of the Reocin MVT Deposit, Spain, by Paleomagnetism
Located in the Basque-Cantabrian basin of northernmost Spain, the Reocin mine exploited one of the worldfs largest known Mississippi Valley-type (MVT) deposits (∼62 Mt at 8.7% Zn and 1.0% Pb) for >100 years. The sphalerite-galena and ferroan dolomite mineralization is in a karst system in 116±1 Ma Urgonian carbonates along the southeastern limb of the Santillana syncline that was formed by late Oligocene and mid Miocene pulses of the Pyrenean Orogeny. Paleomagnetic results from 22 sites (274 specimens) in ore-grade and marginal mineralization isolated a post-folding mid Miocene characteristic remanence (ChRM) direction. Mineral magnetics tests on mineralization, Zn concentrate and tailings show that the ChRM is carried both in sphalerite and ferroan dolomite by pyrrhotite mainly with minor magnetite. We suggest that the deposit was formed in the mid Miocene by fluids flowing from the Pyrenean highlands, ∼50 km SSW of Reocin, that scavenged the metals enroute to Reocin from the Paleozoic basement and/or Mesozoic clastic sediments below the deposit that were derived from the basement.
Spatial Constraints on Carbonate-hosted Base Metals in the Mesoproterozoic Borden Basin, Nunavut
The Milne Inlet Graben is a trough in the aulacogenic Borden Basin, one of several Mesoproterozoic basins that constitute the Bylot Basins. It contains the Nanisivik ore-body as well as numerous other showings spread over an area of 250 x 100 km. Almost all showings are hosted by a dolostone formerly known as the Society Cliffs Formation, which is underlain and overlain by shale. All showings are associated with faults or fracture systems that acted as fluid conduits, but stratigraphic and lithofacies controls are also critical. Revision of stratigraphic units and a new understanding of the tectonostratigraphic dynamics of the basin mean that non- fault-related controls on mineralisation are now more readily characterised. Field mapping and lithostratigraphy have identified three geological settings for Zn±Pb showings. (1) In the northwestern part of the graben, at and near Nanisivik, sphalerite- and pyrite-dominated, replacive mineralisation is present in deep-water dolostone beneath traps formed by shale drapes over erosional highs along an unconformity surface; this the 'gas-cap' mineralisation identified by previous workers. Some of these subtly domal traps are also laterally constrained by downfaulted shale. (2) In the central part of the district, galena-dominated or copper-barite-galena mineralisation is associated with intra-graben fractures and faults, some of which are occluded with 723 Ma gabbro dykes. Mineralisation is concentrated just above the contact of shale and overlying dolostone, immediately below the shale-capped unconformity at the top of the dolostone, or immediately beside a dyke, fault or fracture, at a seemingly random stratigraphic levels. (3) In the southeastern part of the graben, replacive, zinc-rich, lithofacies-controlled mineralisation is present in the immediate vicinity of enormous, graben-bounding, synsedimentarily and postdepositionally active faults. Sulphides are concentrated near the base of the carbonate succession, where they selectively replace only specific shallow-water lithofacies. Ongoing work is aimed at Re-Os dating of the Nanisivik ore deposit. Analytical work to follow will focus on determining whether the showing groups are geochemically or temporally related. The nature of spatial controls on mineralisation in the Borden Basin may contribute to or limit the prospectivity of other aulacogens in the same rift system.
MVT Mineralization in the Maritimes Basin of Eastern Canada: Evidence for Heterogeneity Versus Commonality for Zn-Pb-Ba-Cu-Ag Mineralization
The Carboniferous Maritimes Basin (360,000 sq. km) of eastern Canada is a thick (12 km), dominantly nonmarine, clastic sequence with a Visean-age marine transgression containing marine limestone, evaporites and siltstones (Windsor Group). The lowermost member of this marine succession, the Macumber Fm., hosts significant Pb-Zn-Ba (-Cu-Ag) deposits (e.g., Walton, Gays River, Brookfield, Jubilee). Mineralization is considered to post-date burial and diagenesis of the host rocks by 20-30 Ma, hence it occurred at about 300- 310 Ma. Multi-disciplinary, integrated studies of several mineralized districts have shown that, although mineralization is always at a similar stratigraphic level, where Macumber limestone onlaps basement (Meguma Group) or terrigenous clastics (Horton Group), the features of mineralization vary considerably among deposit sites. The following points highlight this contrasting nature of mineralization and settings: (1) deposits occur above basement highs, at nonmarine clastic-limestone contacts, or fault zones; (2) intense bleaching or alteration of footwall or wallrock lithologies is significant to absent at deposits; (3) presence or absence of pre-ore, diagenetic alteration (i.e., dolomitization, sideritization) of the marine limestone; (4) a range of metal associations, including Zn-Pb-Ba-Cu-Ag, Zn-Pb or Ba, and ratios (Zn:Pb:Cu); (5) presence or absence of syn-ore liquid petroleum and a variable role for the hydrocarbons; (6) different mechanisms for sulphur reduction, as indicated by S isotopic data; (7) a mixture of fluids and reservoirs based on Th-salinity relationships and C-O isotopic data; (8) a range of thermal (100-250°C) and chemical (10-30 wt. % eq. NaCl; range of Na/(Na+Ca)) conditions for fluids through the mineral paragenesis based on fluid inclusion thermometry; and (9) different fluid reservoirs or pathways based on isotopic tracers (Sr, Pb). These data indicate that MVT mineralization in the Nova Scotia segment of the Maritimes Basin is characterized by heterogeneity rather than commonality at a deposit scale with the stratigraphic localization of ore horizons the most apparent similarity. Such heterogeneity of deposit features should be considered when grouping such diverse styles of mineralization into singular deposit types and ore-deposit models, because one deposit model or process may not be applicable to or fit all mineralized sites.
Sr isotopic evidence for fluid mixing in ore-stage dolomites, Pine Point, Northwest Territories, Canada
The carbonate hosted Pb-Zn deposits of the Pine Point district (Northwest Territories) are located close to the eastern edge of the present day Western Canadian Sedimentary Basin. The deposits have been classified as Mississippi Valley Type deposits and are thought to have formed as the result of basin-wide fluid flow in the Presqu'ile barrier, the host to the ore deposits. Laser multi-collector ICP-MS study of 87Sr/86Sr ratios of ore- related dolomites from Pine Point indicate two sources of Sr were present in the mineralizing system. Fluid "A" has a range in Sr isotopic values from 0.07070 to 0.7120 and is a brine derived from Middle Devonian seawater which has undergone some interaction with clastic units in the basin. Fluid "B" has is more enriched in 87Sr and has 87Sr/86Sr ratios up to up to 0.7152, values similar to those found in Canadian Shield Brines, and represents a fluid which has interacted with crystalline basement rocks. The presence of this second Sr source in the ore forming system suggests that sulfide deposition at Pine Point occurred as a result of fluid mixing.
Similarities and Differences between the Sandstone-Hosted Jinding Zn-Pb Deposit and MVT Deposits
The Jinding Zn-Pb deposit (Lanping basin, Yunnan, China) is the largest sandstone-hosted Zn-Pb deposit in the world, having a total reserve of approximately 220 Mt of ore grading 6.1% Zn and 1.3 Pb%. The sedimentary rocks in the Lanping basin were formed in continental environments and were subject to strong deformation during the Himalayan orogeny. The orebodies are hosted in Cretaceous and Paleocene sandstones and pebbly sandstones which formed a structural dome (the Jinding dome) near a regional, high- angle normal fault (the Pijiang fault). The ores can be divided into two types, the sandstone-type and breccia- type. The former consists of fine-grained sphalerite-galena-pyrite-marcasite disseminations in sandstones, and the latter includes sphalerite-galena-pyrite-marcasite disseminations in the matrix and celestite-pyrite- marcasite-sphalerite-galena-calcite filling fractures and cavities. Colloform textures are common in the breccia-type ores, which are associated with sand veins or dykes cemented by sulfides. Breccia-type ores commonly contain solid bitumen, and freshly opened sandstone-type ores have an oily smell. Oil inclusions are common in both types of ores. CO2-CH4-light hydrocarbon inclusions were found in celestite, sphalerite, authigenic quartz, and calcite. Homogenization temperatures of aqueous inclusions range from about 60 to 300 degree C, and salinities range from 1 to 25 wt.% NaCl equivalent. There is a trend of decreasing temperature and increasing salinities away from the Pijiang fault. Delta 34S (CDT) of sulfides range from -32 to 0 per mil. Noble gas isotopes of fluid inclusions and Pb isotopes of sulfides indicate both mantle and crustal sources. It is proposed that the mineralization resulted from mixing between a high-temperature, low-salinity, deep-seated fluid and a relatively high-salinity, low-temperature, basinal fluid. The former ascended along the Pijiang fault and spread westward, and the latter migrated before and during mineralization to the Jinding dome, which was once an oil-gas reservoir. Like Mississippi Valley-type deposits, Jinding is an epigenetic deposit formed from replacement and open- space filling. The association of mineralization with organic matter-rich lithologies, in some cases petroleum reservoirs, is common in MVT deposits. The deposition mechanisms are also similar in that sulfide precipitation resulted from mixing of a metal-rich fluid with a H2S-rich fluid. Therefore, despite the difference in host rocks, the Jinding deposit is comparable to MVT deposits in terms of mineralization styles and deposition mechanisms. On the other hand, Jinding differs from most MVTs in tectonic environments, fluid and metal sources, and driving forces of fluid circulation. Although MVT mineralization can be related to orogenies, the sites of mineralization are generally not tectonically active during mineralization. In the case of Jinding, however, the basin was tectonically active during mineralization, and the site of mineralization was close to an active regional fault and was itself the result of structural activities (thrust faulting and doming). Unlike MVT deposits, where both metals and sulfur were derived from the basin, the Jinding deposit probably derived a major part of its metals from deep-seated, extra-basinal fluids, which may be of mantle origin. Finally, the development of mineralized sand injection veins and dykes in Jinding indicates a forceful, explosive character of mineralization, which is uncommon in MVT deposits.
The Carbonate-Hosted Willemite Deposits in the Zambesi Metamorphic Belt (Zambia): a "Franklin-Type" Mineralization?
The Zambian willemite (Zn2SiO4) deposits occur in metasedimentary carbonate rocks of Proterozoic age. The most important orebodies are located in the dolomites of the Katangan Supergroup at Kabwe, and contain both Zn-Pb sulfides and willemite. The Star Zinc and Excelsior prospects (Lusaka area), discovered in the early twenties and since then subjected to sporadic exploration, are hosted in the highly metamorphic lithotypes of the late Proterozoic Zambezi Supracrustal sequence. In the above-mentioned prospects willemite occurs epigenetically along joints and fissures of the Cheta Fm, consisting mainly of limestone and dolomite marbles, with minor quartz-muscovite schists and feldspathic quartzite. On a local scale, the Star Zinc deposit displays open-space filling, colloform and vuggy textures. Structural analysis resulted in two main fracture trends hosting willemite mineralization: E-W and N-S, the latter being compatible with the Riedel shears related to the pan-African Mwembeshi dislocation zone. Willemite is associated with specular hematite and replaces the Zn- spinels franklinite and gahnite. Calcite commonly replaces willemite. The luminescence of the willemite, observed under cathodic light varies from dull to bright green with recurrent zonations. The green color, however, cannot be attributed to anomalous Mn contents: at Star Zinc Mn is absent in both willemite and franklinite. The Zn-Be bearing sulfosilicate genthelvite [Zn4Be3(SiO4)3S] occurs as a minor phase in irregular aggregates and may contain micrometric inclusions of Fe-Sr-Ba-Fe sulfates. Fluorapatite is also recurrent in the mineral association. At both Star Zinc and Excelsior Zn-Pb sulfides are totally absent, while native silver, as well as traces of germanium and cadmium have been locally detected. Thermometric analyses of willemite inclusions from both prospects result in the following Th: 200-240 °C, and salinities: 8 to 16 wt% NaCl. The zinc spinels franklinite and gahnite are commonly associated with willemite in several hypogene nonsulfide zinc deposits. The typical occurrences are in the Franklin Marbles of the Middle Proterozoic Grenvillian basement in North America, with the best example represented by the Franklin-Sterling Hill deposit. In the latter the franklinite-gahnite-willemite ore association is considered as having been originated by amphibolite-granulite facies metamorphism from a previous zinc sulfide/nonsulfide mineralization. Franklinite (as well as genthelvite) has been reported from the Gamsberg Zn-Pb deposit (Namaqua province, South Africa), which has been also metamorphosed to amphibolite facies. The origin of the franklinite-gahnite protore in the Lusaka area is still unclear. However, the existing mineralization could represent a metamorphosed occurrence derived from primary sulfide concentrations (now completely disappeared), as the Nampundwe massive sulfide deposit occurring SW of Lusaka. Precise age constraints are currently lacking for these willemite deposits. Since no major tectonic deformation is affecting the ores, and there is no record of any geological cover, their only possible temporal constraint may involve the emplacement age of the Hook granite, dated at 559±18 Ma. This intrusion is supposed to be coeval with the Mwembeshi shear zone, whose lineaments are also controlling the Star Zinc deposit.