Magnetism and Iron Oxides

As an IOCG deposit should, the Prospect shows a great amount of magnetism.  To be clear, wherever the words magnetic or magnetism are used on this site, it means the rock attracts a magnet, which indicates a large amount of magnetite.  For a rock to be truly magnetic, it can attract un-magnetized iron.  The ability of a rock to attract a magnet is a measure of its “magnetic susceptibility” but the phrase is not used here.   In the magnetic map of Arizona below, the high magnetism (true magnetism) of the Prospect can be seen at the tip of the arrow.  By zooming, a small light pink area can be seen where the Prospect is located.

A small, but powerful Neodymium magnet attached to a piece of dental floss was used to conduct a magnetic susceptibility survey throughout the Molly Marie Prospect and the results were spectacular. Below shows some of the results of the survey, and there are several more photos of magnetic rock on the Great Mine page.

A 400 to 500 yards wide zone of highly magnetic granite is found surrounding the caldera. The magnet shown is stuck to a vertical face.  This is albite-magnetite alteration.
Albite Magnetite alteration (magnet in center)

Much of the basalt in the main Willow Springs wash and others has very high magnetism.  The magnet by the hammer in this photo  is stuck to a vertical face.

Magnetic Basalt

In the photo below, the altered basalt below the rock hammer is highly magnetic and the basalt above is non-magnetic.  This is evidence of the hydrothermal activity that occurred beneath the basalt.
Lower Alteration zone of Basalt (highly magnetic)

The altered basalt below is highly magnetic, and the magnet is stuck to a vertical face.
Highly magnetic Basalt

The below is Hematite breccia from Cerro Negra in the field.  This likely originated from the boiling zone at the rhyolite porphyry.
Massive Hematite Breccia

The below is from a hematite breccia pipe from Cerro Negra.  It is very slightly magnetic.  Note: hematite by itself is not magnetic.
Sawn Hematite Breccia

The rock below has veinlets of silica and magnetite and is from a breccia pipe in Area 1.  The wider veinlet is very magnetic.
Magnitite Veinlets

This breccia is from the volcanic neck of the caldera, and it is more magnetic than the red rhyolite porphyry.
Rhyolite Breccia

This is a hematitic breccia from near the base of Cerro Negra.  It’s magnetism is unknown.
Hematite Breccia

This is exhalite from area 1, it high in hematite, and its magnetism was not tested.
Saccharoidal silica gossan

This banded Chert shown below, in Area 3 is magnetic.
Banded Chert

This slab of banded chert is magnetic.
Sawn Banded Chert

This breccia below from area 1 is magnetic. When magnified, tiny stringers of magnetite can be seen.
Chert Breccia with abundant Magnetite

The aquifer (Arkose) hosted jasperoid beds outcropping on the east side of Area 1 are sometimes slightly magnetic, but are most often non-magnetic.  The jasperoid is hematite and silica that emanates from the boiling zone above the Rhyolite porphyry.

Jasperoid beds in the Arkose.