HR: 0800h
AN: GP41A-0856    [Abstracts]
TI: Rock Magnetic Properties of Samples From El Laco Magnetite Flow, High Andes, Chile, With Particular Attention to High Coercivity Materials
AU: * Brown, L L
EM: lbrown@geo.umass.edu
AF: Dept. Geosciences, Univ. Massachusetts, Amherst, MA 01003 United States
AU: McEnroe, S A
EM: suzanne.mcemroe@ngu.no
AF: Geol Survey Norway, Leiv Eirikssons vei 39, Trondheim, N-7491 Norway
AU: Harrison, R J
EM: rjh40@esc.cam.ac.uk
AF: Dept Earth Sciences, Univ. Cambridge, Cambridge, CB2 3EQ United Kingdom
AB: El Laco magnetite flow, located on the crest of the Andes of northern Chile, has been extensively mined for iron ore. Samples of ore contain up to 90% iron oxides, mostly in the form of magnetite, with isolated occurrences of hematite and maghemite. Formation of this unique deposit is controversial and has been interpreted as the result of hydrothermal activity, metasomatic replacement or original volcanic deposits of iron-rich magma. Oriented samples were collected from five sites within the ore bodies as well as two sites in the adjacent andesitic flows, along with float samples from the extensive mine dumps. Susceptibility values fall into two groups, with 75% of the samples having susceptibilities greater than 0.1 SI (mean value of 0.305) and 25% having susceptibilities of less than 0.02 (mean value of 0.011). Surrounding andesitic lavas have a mean susceptibility of 0.035 SI). Hysteresis properties are also skewed into two distinct groups, with most samples having coercivities less than 60mT, but with one distinct group of Hc values greater than 100 mT and as large as 440mT. Pinched-waist loops with initial low coercivities have coercivities greater than 100mT above 580C. The high coercivity material is the primary subject of this study. Measurements at high and low temperature (970K to 10K) of hysteresis properties, temperature-susceptibility and remanence measurements are evaluated in the light of TEM studies. TEM results reveal that the high coercivity material is an ultra-fine intergrowth of hematite and maghemite. Many maghemite particles are only unit cell size. Exchange coupling between the maghemite and hematite phases has been observed in off-axis electron holography (TEM).
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005