Properties

Magnetic lows may be associated with the mineralized zones where original magnetite is destroyed by the alteration. Acid weathering of the pyrite contributes to clay alteration and increases near-surface conductivity. The unmineralized gradodioritic-porphyry rocks are expected to be more resistive and moderately magnetic; whereas the unmineralized volcanic andesite-basalt rocks are expected to be moderately resistive and highly magnetic. A tropical saprolite overburden (up to 30 m) exists, which is highly conductive, and transition to resistive host rock occurs rapidly over a few meters of saprock. Between mineralized and unmineralized, there are significant physical property contrasts: higher conductivity and lower susceptibility when mineralized. Furthermore, for an EM method, conductive saprolitic overburden over the deposit can create challenges due to reduced depth of investigation and potential current channeling effects.

Table 1: Below is a summary of geophysical of the Balboa responses that is consistent with the work by [Hos10] on porphyry copper-gold systems.

Rock Unit	Resistivity (\(\Omega \cdot m\))	Susceptibility (SI)
Saprolitic overburden	Low	Low
Host rock	High	Low
Granodiorite/porphyry (host rock; unmineralized)	Moderate	Moderate
Andesite/basalt (unmineralized)	Moderate	High
Mineralized/clay-altered	Low	Low