Paleomagnetism and Rock Magnetism of 'SEDEX' Zinc-Lead-Copper Ores in Black Shales in Australia and Yukon and of Fluorite Veins in Granite in Newfoundland.

摘要

The paleomagnetic age dating technique is used to study the genesis of a fluorite and four "sedimentary exhalative (SEDEX)" deposits which are hard to date by conventional radiometric methods because they lack suitable minerals, which causes controversy about their ore genesis. Paleomagnetic analysis of 359 specimens from the Devonian St. Lawrence Granite and fluorite veins in Newfoundland shows that the granite and fluorite veins are coeval and retain a primary remanence carried by pyrrhotite and/or magnetite. The difference between the measured and expected paleopoles is attributed to postemplacement counterclockwise rotation of ∼17° of the area about a vertical axis during the Late Devonian Acadian Orogeny. Paleomagnetic analysis of 333 specimens from the Century Zn-Pb-Ag deposit in Australia isolates a stable characteristic remanent magnetization (ChRM) for ore sites only that resides mainly in single (SD) or pseudosingle domain (PSD) titanomagnetite and pyrrhotite. A positive fold test shows that the ore's ChRM predates D2 deformation in the Mesoproterozoic Isan Orogeny. The paleomagnetic age of 1558+/-4 Ma supports a late diagenetic replacement model for ore genesis. Paleomagnetic and rock magnetic analyses of 333 specimens from the Mount Isa Zn-Pb-Cu-Ag and George Fisher Zn-Pb-Ag deposits in Australia isolate a stable ChRM carried mainly by SD or PSD pyrrhotite and/or titanomagnetite. The negative fold test shows that the ChRM postdates the ∼1510 Ma D3 deformation in the Isan Orogeny. Thus the ∼1505 Ma paleomagnetic age provides a minimum age for the ores and an age for the greenschist metamorphism during the orogeny. Paleomagnetic and rock magnetic analyses of 339 specimens from six mineralized panels in the Howards Pass Zn-Pb deposits in Yukon isolate a stable ChRM that mainly resides in SD or PSD pyrrhotite and titanomagnetite. The paleopole falls on Middle Jurassic portion of the North American apparent polar wander path (APWP) or corrected APWP for the Intermontane terranes, giving ∼170 Ma and ∼162 Ma, respectively. The results indicate that the Selwyn Basin had undergone ∼35° clockwise rotation with ∼10° northward translation between Middle Jurassic and mid-Cretaceous.