5.2 Igneous, Metamorphic & Sedimentary Ore Mineralogy

Mineralogy and petrology form the core foundation of economic geology. Identifying ore and gangue minerals, understanding their paragenesis, and interpreting hydrothermal alteration patterns allow mining engineers and geologists to map deposit zoning, optimize metallurgical recovery, and execute effective grade control.

Major Rock-Forming & Ore-Forming Minerals

Economic mineral deposits consist of ore minerals (minerals from which metals can be extracted at a profit) and gangue minerals (non-valuable mineral matrix surrounding the ore).

Primary Sulfide & Oxide Ore Minerals

• Chalcopyrite ($\text{CuFeS}_2$): The primary copper ore mineral globally and in Philippine porphyry systems. Characterized by a brass-yellow color, greenish-black streak, Mohs hardness of $3.5\text{--}4.0$, and tetrahedral crystal habit.
• Bornite ($\text{Cu}_5\text{FeS}_4$): Known as "peacock ore" due to iridescent purplish-blue tarnish. High copper content ($\sim63.3%\text{ Cu}$), indicating high-grade hypogene ore zones.
• Galena ($\text{PbS}$): Primary lead ore, commonly bearing silver ($Ag$). Recognizable by lead-gray color, bright metallic luster, high specific gravity ($SG = 7.5\text{--}7.6$), and perfect $90^\circ$ cubic cleavage.
• Sphalerite ($\text{ZnS}$): Primary zinc ore. Exhibits resinous to adamantine luster, yellow-brown to black color depending on iron content, Mohs hardness $3.5\text{--}4.0$, and perfect six-direction dodecahedral cleavage.
• Pyrite ($\text{FeS}_2$): Ubiquitous iron sulfide ("fool's gold"). While rarely an economic ore itself, its abundance and trace element content make it a critical pathfinder mineral and indicator of hydrothermal alteration halos.
• Chromite ($\text{FeCr}_2\text{O}_4$): Oxide mineral of the spinel group, primary source of chromium. Submetallic black color, brown streak, high specific gravity ($4.5\text{--}4.8$), and octahedral crystal form. Hosted in ultramafic rocks.

Nickeliferous Laterite Minerals

In Philippine weathered ophiolite profiles, nickel is concentrated into two distinct zones:

1. Goethite ($\alpha\text{-FeO(OH)}$): Dominates the upper Limonite Zone. Fine-grained, earthy reddish-brown iron oxyhydroxide hosting $0.8\text{--}1.2%\text{ Ni}$ adsorbed onto iron hydroxides, along with cobalt credits ($0.05\text{--}0.15%\text{ Co}$).
2. Garnierite: A general term for hydrous nickel-magnesium silicates (including népouite, pimelite, and nickeliferous serpentine, talc, or smectite) in the lower Saprolite Zone. Distinctive apple-green color, soft texture, high nickel grade ($1.5\text{--}2.5%\text{ Ni}$).

Igneous Petrology & Tectonic Environments

Magmatic processes dictate the metal endowment of island arcs. Two geological associations are especially relevant to Philippine metallogeny:

Calc-Alkaline & Adakitic Arc Magmatism

Porphyry copper-gold deposits are intimately linked to subduction-related calc-alkaline diorite, quartz diorite, granodiorite, and dacite intrusives. Where young, hot oceanic lithosphere subducts, or where lower crust melts at high pressures, adakitic magmas are generated. Rocks described as adakitic commonly show elevated $\text{Sr/Y}$ and $\text{La/Yb}$ with depleted heavy rare earth elements, but numeric thresholds and petrogenesis are context-dependent. Oxidation state and magmatic water also require direct mineralogical or geochemical evidence rather than inference from one ratio. In some fertile systems, oxidized hydrous magma can delay sulfide saturation and retain copper, gold, and sulfur for later magmatic-hydrothermal transfer. This is a process hypothesis tested with petrology, mineral chemistry, inclusions, and whole-rock data, not a consequence of an “adakite” label alone.

Ultramafic Ophiolite Suites

Ophiolites are tectonic slabs of oceanic mantle and crust emplaced onto continental or island-arc margins. A complete ophiolite sequence comprises (from base to top): mantle peridotite (harzburgite, dunite), layered gabbros, sheeted dike complexes, pillow basalts, and pelagic sediments (e.g., Zambales, Angat, Palawan, and Surigao Ophiolites). Podiform chromite is associated with dunite-rich mantle conduits where melt-rock reaction, crystallization, and segregation concentrate chromian spinel; it is not explained by simple crystal settling alone. Deep tropical weathering of ultramafic peridotites can yield nickel laterite deposits.

Hydrothermal Alteration Assemblages & Zoning

Hydrothermal fluids react with wall rocks, replacing primary minerals with secondary alteration assemblages. Alteration zoning provides a spatial vector toward the center of mineralization.

[ Distal: Propylitic ] ---> [ Intermediate: Phyllic ] ---> [ Central Core: Potassic ]
(Chlorite, Epidote, Calcite)   (Quartz, Sericite, Pyrite)    (K-Feldspar, Biotite, Magnetite)

Classic Porphyry Alteration Zones

1. Potassic Zone: High-temperature ($350^\circ\text{C to }>600^\circ\text{C}$) central core formed by hypersaline magmatic fluids. Characterized by secondary K-feldspar, hydrothermal biotite, magnetite, and quartz stockworks. Hosts main-stage chalcopyrite-bornite mineralization.
2. Phyllic (Sericitic) Zone: Moderate-temperature ($200^\circ\text{C to }350^\circ\text{C}$) overprint by acidic fluids. Diagnostic minerals: Quartz, sericite (fine-grained muscovite), and abundant coarse pyrite (the "pyrite halo").
3. Argillic Zone: Moderate to low-temperature ($100^\circ\text{C to }250^\circ\text{C}$) acidic alteration. Characterized by clay minerals such as kaolinite, smectite, and illite.
4. Advanced Argillic Zone: Extreme low-pH ($<2.0$) hypogene leaching by condensate magmatic vapor. Diagnostic minerals: Vuggy quartz, alunite, pyrophyllite, dickite, and native sulfur. Typical host for high-sulfidation epithermal Cu-Au ores (e.g., Lepanto Mankayan).
5. Propylitic Zone: Commonly distal alteration involving chlorite, epidote, calcite, albite, and minor pyrite under broadly near-neutral conditions.

The listed temperatures and spatial order are teaching ranges for an idealized model. Assemblages overlap, are overprinted, and depend on pressure, fluid chemistry, host rock, permeability, and paragenesis; mineral equilibrium or fluid-inclusion evidence is needed for a project temperature.

Physical Mineral Identification Techniques

Field identification relies on simple diagnostic physical properties:

• Mohs Hardness: Pyrite ($6.0\text{--}6.5$) easily scratches a steel knife blade ($5.5$), whereas chalcopyrite ($3.5\text{--}4.0$) is readily scratched by steel.
• Specific Gravity (SG): Galena ($7.5\text{--}7.6$) and chromite ($4.5\text{--}4.8$) are readily distinguished from silicate gangue ($SG \approx 2.65$) by weight estimation.
• Cleavage vs. Fracture: Galena displays perfect three-direction cubic cleavage ($90^\circ$), while chromite exhibits uneven, sub-conchoidal fracture.
• Streak: Pyrite yields a brownish-black streak, chalcopyrite a greenish-black streak, goethite a yellowish-brown streak, and hematite a cherry-red streak.