Abstract

The hematite and pyrrhotite ores of the Chichibu pyrometasomatic deposits have reversed polarity magnetizations. Among late Miocene igneous bodies a pluton named the quartz diorite C body has a similar magnetization direction and polarity. Integrating paleomagnetic data and opaque mineralogical observations, it is determined that the ore deposits (n = 26, D = 197 degrees , I = -54 degrees , alpha = 12 degrees ) have a genetic relationship to the quartz diorite C body (fresh sites: n = 15, D = 211 degrees , I = -57 degrees , alpha = 12 degrees ; altered sites: n = 19, D = 190 degrees , I = -57 degrees , alpha = 9 degrees ) rather than to the quartz diorite A body (fresh sites: n = 33, D = 023 degrees , I = +57 degrees , alpha = 6 degrees ; extremely altered sites: n = 6, D = 039 degrees , I = +52 degrees , alpha = 14 degrees ) or the quartz diorite B body (extremely altered sites: n = 7, D = 306 degrees , I = +56 degrees , alpha = 4 degrees ).The timing of formation of the Chichibu pyrometasomatic deposits is given chronologically: (1) intrusion of the quartz diorite A body in the normal polarity chron; acquisition of a normal polarity magnetization by the A body; (2) intrusion of the quartz diorite C body after the change of the geomagnetic field from normal to reversed; acquisition of a reversed polarity magnetization by the C body; (3) skarn formation, mineralization, and hydrothermal alteration in the reversed chron; acquisition of reversed polarity magnetization by hematite and pyrrhotite ores; (4) intrusion of dike rocks just after the main mineralization during the reversed polarity chron; acquisition of a reversed magnetization by dike rocks; and (5) extremely strong alteration with pyritization after the change of the geomagnetic field from reversed to normal; acquisition of normal polarity magnetization by the quartz diorite B body and a part of the quartz diorite A body.Moreover, it is demonstrated that the time sequence from the intrusion of the related rock, that is, the quartz diorite C body, to the end of main mineralization of these ore deposits falls within one reversed polarity chron of the late Miocene. Its span is on the order of 10 5 years.

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