Paleomagnetic dating of Alleghanian orogenesis and mineralisation in the Mascot–Jefferson City zinc district of East Tennessee, USA

The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (d_p=18°, d_m=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (d_p=5.3°, d_m=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (d_p=2.4°, d_m=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.     

Paleomagnetism of the triassic nicola volcanics and geotectonics of the quesnellia subterrane of terrane I, British Columbia

Paleomagnetic data from 46 sites (674 specimens) of the Westcoast Crystalline Gneiss Complex on the west coast of Vancouver Island using AF and thermal demagnetization methods yields a high blocking temperature WCB component (> 560°C) with a pole at 335°W, 66°N (δ_p = 4°, δ_m = 6°) and a lower coercivity WCA component ( 25 mT, < 500°C) with a pole at 52°W, 79°N (δ_p = 7°, δ_m = 8°). Further thermal demagnetization data from 24 sites in the Jurassic Island Intrusions also defines two high blocking temperature components. The IIA component pole is at 59°W, 79°N (δ_p = 7°, δ_m = 8°) and IIB pole at 130°W, 73°N (δ_p = 12°, δ_m = 13°). Combined with previous data from the Karmutsen Basalts and mid-Tertiary units on Vancouver Island and from the adjacent Coast Plutonic Complex, the geotectonic motions are examined for the Vancouver Island segment of the Wrangellian Subterrane of composite Terrane II of the Cordillera. The simplest hypothesis invokes relatively uniform translation for Terrane II from Upper Triassic to Eocene time producing 39° ± 6° of northward motion relative to the North American craton, combined with 40° of clockwise rotation during the Lower Tertiary.     

Current-scattered (?) detrital remanence directions in the Zn-rich Pennsylvanian Stark black shale, USA

Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes.     

Foraminiferal associations of the Mid-Tertiary Edegem Sands at Terhagen, Belgium

A factor-analytic approach (varimax rotated factors) was used in studying the benthonic foraminiferal associations of the Mid-Tertiary Edegem Sands at Terhagen (near Antwerpen) Belgium. The fauna can be described by two groups. The lower and upper part of the stratigraphic section are characterized by one group and the middle transitional part by a second group. The results can be correlated with the number of the planktonic forms and with the fossil richness of the sediment.     

Paleomagnetic constraints on Zn–Pb ore genesis of the Pillara Mine, Lennard Shelf, Western Australia

The Pillara Zn–Pb deposit is the largest of several known Mississippi Valley-type (MVT) deposits in the Lennard Shelf of the Canning Basin. Paleomagnetic and rock magnetic measurements are reported for 294 specimens from 23 sites in mineralization and its carbonate host rocks from the deposit as well as on 15 artificial specimens of zinc and lead concentrate and of tailings. Pyrrhotite carries the characteristic remanent magnetization (ChRM) in nearly all specimens. The ChRM postdates most faulting as shown by breccia tests and most minor regional tilting as shown by the degraded fit on tilt correction. The mean ChRM direction for all sites is D=20.6°, I=–27.5° (N=23, ₉₅=5.3°, k=34.1), yielding an age of 358±5 Ma (2) that is similar to the comparable age of 354±8 Ma (2) for the Kapok MVT deposit. Host rock diagenesis with attendant secondary remagnetization yields an age of 361±5 Ma (1) and the MVT mineralization with a primary chemical remanent magnetization gives an age of 356±3 Ma (1), co-eval with a published Rb–Sr sphalerite age of 357±3 Ma. Interpretation of this temporal data suggests that the MVT deposits of the southeastern Lennard Shelf originated during extension, probably in response to rift-related topography-driven fluid flow.Editorial handling: C. Brauhart     

Paleomagnetism of the Society Cliffs dolostone and the age of the Nanisivik zinc deposits, Baffin Island, Canada

Dolostones of the ∼1200 Ma Society Cliffs Formation within the hydrothermal zone surrounding the Nanisivik zinc deposits retain a stable characteristic remanent magnetization (ChRM) on alternating field and thermal step demagnetization. Based on the thermal data and saturation isothermal remanence analyses, the ChRM resides in pseudosingle domain magnetite and hematite. A paleomagnetic fold test favours a post-folding ChRM, and a paleomagnetic contact test, using a Franklin gabbro dike, indicates that the ChRM predates ∼720 Ma. The pole position calculated from the ChRM direction is at 168.2°E, 42.8°N (δ_p=4.9°, δ_m=6.8°), giving an age of 1095 ± 10 Ma on the well-defined “Logan Loop” portion of the North American apparent polar wander path. This age is considered to date recrystallization of the dolostone host rocks in the halo around the hydrothermal sulfide deposits. No evidence is found for a postulated Cretaceous remagnetization event in the region. Received: 9 January 1999 / Accepted: 3 March 2000     

Palaeomagnetic evidence of a Variscan age for the epigenetic Galmoy zinc–lead deposit,Ireland

Palaeomagnetism of 273 specimens from 24 sites isolated a well‐defined characteristic remanent magnetization (ChRM) direction on AF and thermal demagnetization in seven host carbonate and 14 ore mineralization sites from the Galmoy Zn–Pb deposit. Thermal decay and saturation remanence data show that the ChRM is carried dominantly by single domain magnetite. Palaeomagnetic field stability tests indicate a post‐brecciation and post‐folding ChRM. The ChRM directions from the host rock and mineralized sites are indistinguishable at 95% confidence and give a palaeopole at 41.5°S, 8.4°W (dp = 1.5°, dm = 3.0°) with an age of 290 ± 9 Ma on the Laurentian apparent polar wander path. This Early Permian age at Galmoy records Variscan orogenesis and suggests an epigenetic model in which mineralization occurred during cooling from the regional Variscan thermal episode.     

Brigantian and Middle Triassic remagnetizations in Lower Carboniferous carbonates, Northern Ireland: Role of diagenesis and fluid flow

Paleomagnetism (18 sites, 231 specimens) of Lower Carboniferous carbonates in Northern Ireland reveals three characteristic remanent magnetization (ChRM) components. Six sites from Brigantian limestones have a Middle Triassic (239 ± 7 Ma) secondary chemical remanent magnetization (CRM) in hematite, likely from alteration of the limestones by oxidizing meteoric fluids when continental red beds were deposited immediately above. Twelve sites from early Asbian limestones retain ChRM directions residing in pyrrhotite and magnetite. Their paleopoles are statistically indistinct, but suggest that the pyrrhotite remanence (326 ± 4 Ma) is about a million years younger than the magnetite remanence (327 ± 3 Ma). More importantly, the primary ChRM in these limestones was reset 3 or 4 Ma after deposition, probably by fluids involved in their diagenesis, giving secondary CRMs that are 8 Ma younger than those observed in the Lower Carboniferous carbonates that host the Navan Zn–Pb deposit in the Irish Midlands, suggesting two unrelated fluid histories.     

Mississippi Valley-type lead–zinc deposits through geological time: implications from recent age-dating research

Remarkable advances in age dating Mississippi Valley-type (MVT) lead-zinc deposits provide a new opportunity to understand how and where these deposits form in the Earth's crust. These dates are summarized and examined in a framework of global tectonics, paleogeography, fluid migration, and paleoclimate. Nineteen districts have been dated by paleomagnetic and/or radiometric methods. Of the districts that have both paleomagnetic and radiometric dates, only the Pine Point and East Tennessee districts have significant disagreements. This broad agreement between paleomagnetic and radiometric dates provides added confidence in the dating techniques used. The new dates confirm the direct connection between the genesis of MVT lead-zinc ores with global-scale tectonic events. The dates show that MVT deposits formed mainly during large contractional tectonic events at restricted times in the history of the Earth. Only the deposits in the Lennard Shelf of Australia and Nanisivik in Canada have dates that correspond to extensional tectonic events. The most important period for MVT genesis was the Devonian to Permian time, which corresponds to a series of intense tectonic events during the assimilation of Pangea. The second most important period for MVT genesis was Cretaceous to Tertiary time when microplate assimilation affected the western margin of North America and Africa-Eurasia. There is a notable paucity of MVT lead-zinc ore formation following the breakup of Rodinia and Pangea. Of the five MVT deposits hosted in Proterozoic rocks, only the Nanisivik deposit has been dated as Proterozoic. The contrast in abundance between SEDEX and MVT lead-zinc deposits in the Proterozoic questions the frequently suggested notion that the two types of ores share similar genetic paths. The ages of MVT deposits, when viewed with respect to the orogenic cycle in the adjacent orogen suggest that no single hydrologic model can be universally applied to the migration of the ore fluids. However, topographically driven models best explain most MVT districts. The migration of MVT ore fluids is not a natural consequence of basin evolution; rather, MVT districts formed mainly where platform carbonates had some hydrological connection to orogenic belts. There may be a connection between paleoclimate and the formation of some MVT deposits. This possible relationship is suggested by the dominance of evaporated seawater in fluid inclusions in MVT ores, by hydrological considerations that include the need for multiple-basin volumes of ore fluid to form most MVT districts, and the need for adequate precipitation to provide sufficient topographic head for topographically-driven fluid migration. Paleoclimatic conditions that lead to formation of evaporite conditions but yet have adequate precipitation to form large hydrological systems are most commonly present in low latitudes. For the MVT deposits and districts that have been dated, more than 75% of the combined metal produced are from deposits that have dates that correspond to assembly of Pangea in Devonian through Permian time. The exceptional endowment of Pangea and especially, North America with MVT lead-zinc deposits may be explained by the following: (1) Laurentia, which formed the core of North America, stayed in low latitudes during the Paleozoic, which allowed the development of vast carbonate platforms; (2) intense orogenic activity during the assembly of Pangea created ground preparation for many MVT districts through far-field deformation of the craton; (3) uplifted orogenic belts along Pangean suture zones established large-scale migration of basin fluids; and (4) the location of Pangea in low latitudes with paleoclimates with high evaporation rates led to the formation of brines by the evaporation of seawater and infiltration of these brines into deep basin aquifers during Pangean orogenic events.