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, by strict definition, it attracts 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 silica mounds cut by 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 silica below the rock hammer is highly magnetic and the basalt above is non-magnetic.  The silica mounds often show this phenomena.  Early in their life they were much more magnetic and late in their life, less magnetic and they contain much more carbonates.
Lower Alteration zone of Basalt (highly magnetic)

The silica 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.  It is slightly magnetic.
Massive Hematite Breccia

The below is from a hematite breccia pipe from Cerro Negra with some clasts of basalt.  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 slight.
Hematite Breccia

This is exhalite from area 1, it is high in hematite, and its magnetism is weak.
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 sawn breccia below from area 1 is magnetic. When magnified, tiny stringers of magnetite can be seen. The delicate depositional bands of silica with magnetite can be seen in this sample.   This sample speaks volumes of what lies deeper in Area 1.
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 below the IOCG deposit.

Jasperoid beds in the Arkose.