GP41B-01
Paleomagnetism of the Zn-rich Pennsylvanian Stark black shale, Kansas City area, U.S.A.
Paleomagnetic results are reported from the metalliferous Stark black shale (Heebner-type) in the Upper Pennsylvanian Kansas City Group. Paleomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization (ChRM) in 17 sites that yields a Late Mississippian to Middle Pennsylvanian paleopole at 32.2 ° N 128.5 ° E (dp = 4.7° and dm = 8.8°). The fact that the observed age is slightly older than the host rock age indicates that the shalefs mineralization has a syngenetic origin. The main remanence carrier in the Stark Shale is single or psudosingle domain magnetite or titanomagnetite. Trace hematite, which was generated during modern weathering, likely causes the slightly older age by steepening the primary ChRM by ~2°. The large oval of 95 % confidence is interpreted to be caused by the formation of clay-magnetite aggregates during sediment transport that are easily biased by the gentle paleocurrent at each site acting on their large flat surface. 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 and not of remagnetization by secondary hydrothermal processes. The finding that the primary ChRM directions of Heebner-type black shales are so easily scattered by sedimentary processes implies that a highly clustered ChRM direction in a mineralized black shale, such as in Zn-Pb rich SEDEX deposits, is evidence of coeval or post-depositional hydrothermal fluids.
GP41B-02
Magnetic Properties of Mineralized Rocks in El Teniente Porphyry Copper Deposits, Central Andes, Chile
El Teniente porphyry copper deposit, the world's greatest intrusion related Cu-Mo orebody (central Chile) is hosted within basaltic-andesitic volcanic and gabbroic rocks. This orebody is strongly affected by multiple events of alteration/mineralization with pervasive potassic and chloritic alteration. The mineralization is induced principally by felsic apophyses and dykes (quartz diorite to tonalite) and porphyritic stocks, with U/Pb ages between 7-2.8 Ma, and occurs as a stockwork of mineralized veins and hydrothermal breccias. Alteration biotite and sericite minerals have 40Ar/39Ar ages between 6.3-4.4 Ma. We have undertaken paleomagnetic sampling of the deposit using mining drill cores and oriented block samples to test the possibility of a post-mineralization deformation and tilt. Taking into account the large size and the young age of this giant deposit, the aims of the paleomagnetic study are also to characterize the magnetic properties and better constrain the timing and duration of the mineralization events. Magnetic experiments (hysteresis loop, IRM, k-T curves, thermal and AF demagnetization) show the presence of prevailing magnetite. Microscope and SEM observations and chemical analyses suggest several magnetite families. Large multidomain magnetite grains, associated with biotite and chlorite are related to different hydrothermal alteration pulses. However, plagioclases showing an early Na-Ca-Fe alteration contain numerous small (1-10μm) to very small magnetites (<1μm). While the felsic intrusions have low magnetic susceptibility (<0.0005SI) and low NRM, the mineralized mafic rocks have usually high susceptibility (>0.01 and up to 0.2SI) with NRM in the range 0.1- 2A/m. Most samples have univectorial magnetizations with high MDF values (>20mT) and high unblocking temperatures (500-580°C). Within the mine the magnetic polarity is spatially distributed and associated to different stocks. Variations of the magnetic polarity indicate different mineralization events occurred in time and space within the deposit. There is no paleomagnetic evidence for post-mineralization deformation.
GP41B-03
Magnetic Layering in the Great Dyke of Zimbabwe: Implications for Emplacement and ore Genesis
The Great Dyke of Zimbabwe (GDZ) is an elongated mafic-ultramafic layered complex (3-11 km x 550 km) emplaced at 2575 ± 5 Ma in the Archean Zimbabwe Craton. It consists of 5 linked layered subchambers with a funnel shape in cross-section. The igneous layering in the Great Dyke dips gently inward to the center of each subchamber. All subchambers exhibit similar stratigraphy and consist of a lower ultramafic series capped by gabbros. The GDZ was formed by emplacement of successive magma batches that differentiated in-situ. Magmatic fractionation and mixing processes resulted in the development of a prominent compositional layering and ore concentration. Specimens come from a 592 m-long borehole core drilled through the mafic and ultramafic sequences. The magnetic susceptibility (K) was measured in low-field (LF) at a 1.5 m spacing, and in high-field (HF) at a 7.5 m spacing. KLF ranges from 113 to 100000 x 10-6 [SI], while KHF ranges from 109 to 979 x 10- 6 [SI]. The variation of KLF and KHF allows for the identification of ferromagnetic and paramagnetic contributors to K. The mode of ferromagnetic oxides varies periodically and defines several layers with high magnetite content. The paramagnetic contributors show a slow increase through the lower mafic rocks. One section of the core in the lower mafic sequence displays anomalous magnetic characteristics such as a large increase in both low and high field magnetic susceptibility, and a decrease in mineral grain size. The magnetic results clearly discriminate rocks of the lower mafic sequence, ultramafic sequence, and rocks that are likely related to a later intrusive event into the GDZ. Additionally, the magnetic data allows for the further subdivision of the lower mafic sequence into magnetic layers with distinct characteristics. The disruption of all magnetic properties at the mafic-ultramafic boundary suggests a dramatic change in crystallization conditions. The slow increase of paramagnetic contributors in the lower mafic sequence reveals a slower change in crystallization conditions. This data seems to suggest a disturbance at the mafic-ultramafic boundary, possibly a reintrusion or extrusion event, followed by undisturbed fractional crystallization in the lower mafic sequence. This information has important implications for the magmatic history of this part of the magmatic chamber, and also for formation of major sulfide ore deposits located a few meters below the mafic-ultramafic boundary. Additional information about the magmatic history near this major boundary could help determine if these ore deposits result from cumulus settling of immiscible sulfide liquid, or by later post-cumulus processes involving late-stage magmatic fluids.
GP41B-04
Paleomagnetic Determination of Pre-Mining Metal Flux Rates at the Iron Mountain Superfund Site, Northern California
Iron Mountain, located near Redding in northern California, hosts a group of mines that were active from the late 1870s to the early 1960s. The mineral deposit is classified as a type-I volcanogenic massive sulfide, similar to the Noranda deposit of Ontario, Canada. Three large, isolated blocks of sulfide mineralization contain 90-95 percent pyrite and a few percent chalcopyrite (CuFeS2) and sphalerite (ZnS). Prior to mining, weathering converted parts of the massive sulfide to gossan consisting of hematite, goethite, and silica. Mining further exposed the pyritic masses to water and air, creating optimal conditions for sulfide oxidation and production of acid mine drainage. Because the acidic, metal-rich effluent reached the Sacramento River, the site has been one of the highest priorities on the US EPA's Superfund list since the early 1980s. A crucial area of scientific uncertainty that needed to be resolved was the magnitude of natural background metal flux. We collected 25 paleomagnetic samples from the gossan to determine the polarity of the Earth's magnetic field during pre-mining sulfide weathering. Nineteen samples exhibited stable magnetic endpoints during thermal demagnetization; of these, four were of reversed polarity and the remainder were of normal polarity. This result established that the gossan was already forming 780,000 years ago, and this information made it possible to estimate natural, pre- mining flux rates of copper and zinc. These rates were three orders of magnitude lower than post-mining (pre- remediation) rates. Resolution of the question of the background flux led to one of the largest legal settlements in U.S. history for remediation of an inactive mine site.