6.1 Geochemical & Remote Sensing Exploration Techniques

Mineral exploration is a high-risk, capital-intensive endeavor that aims to discover economically viable mineral deposits beneath the Earth's surface. To systematically reduce technical and financial uncertainty, exploration programs follow a staged workflow ranging from broad regional reconnaissance to site-specific deposit evaluation. Geochemical surveying and spectral remote sensing represent the primary front-line methodologies deployed during early-stage exploration to identify hydrothermally altered rock packages and surface geochemical anomalies.

Staged Mineral Exploration Framework

Modern mineral exploration programs progress through four sequential stages, with each stage serving as a decision gate to commit or divest capital:

1. Regional Reconnaissance (Grassroots Exploration): Covers large areas ($1,000\text{ to } 10,000\text{ km}^2$) to establish regional metallogenic potential. Activities focus on satellite imagery interpretation, airborne geophysics, and wide-spaced stream sediment sampling.
2. Target Generation (Prospecting): Reduces exploration area to $10\text{ to } 100\text{ km}^2$. High-resolution remote sensing, grid-based soil sampling, and ground geophysical surveys delineate anomalous zones for follow-up.
3. Detailed Prospect Evaluation: Encompasses $1\text{ to } 10\text{ km}^2$. Outcrop mapping, structural analysis, trenching, rock chip sampling, and initial scout drilling test mineralized structures at depth.
4. Deposit Delineation & Pre-Feasibility: May involve progressively closer-spaced diamond drilling, detailed core logging, 3D geological and grade modeling, QA/QC verification, and metallurgical testing to estimate mineral resources under recognized reporting codes (e.g., PMRC, JORC, NI 43-101).

The stage areas above are order-of-magnitude illustrations. Deposit footprint, access, terrain, data maturity, decision, and budget determine program scale; stages can overlap and targets can be abandoned or recycled.

Geochemical Exploration Protocols & Media Selection

Geochemical exploration measures the abundance and distribution of chemical elements in natural surface materials to detect primary and secondary dispersion halos.

1. Stream Sediment Sampling & BLEG

Stream sediment sampling evaluates active drainage catchments by collecting fine sediment deposited by running water. In mountainous tropical catchments, stream sediment surveys provide rapid coverage of entire river basins.

• Conventional Stream Sediment Surveys: Sediment is sieved in the field to fine fractions (typically $-80\text{ mesh}$ or $177\text{ }\mu\text{m}$) and analyzed for multi-element suites using Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
• Bulk Leach Extractable Gold (BLEG): BLEG leaches a comparatively large stream-sediment sample to reduce nugget-effect variability and improve sensitivity to low-level catchment anomalies. Sample mass, preparation, leach chemistry, reporting limit, and downstream dispersion distance must be established by the program and laboratory rather than assumed from one generic value.

2. Soil Geochemistry & Sampling Grids

Soil sampling maps secondary dispersion halos in residual soil profiles overlying bedrock mineralization.

• Horizon Selection: Select and document a consistent sampling medium only after an orientation survey relates horizons, regolith processes, and target response. A B-horizon may concentrate metals on clays or iron-manganese oxides, while organic A-horizon material is often avoided and saprolite may be appropriate for another program; no horizon is universally discarded or preferred.
• Grid Patterns: Grid orientation and spacing follow expected target geometry, dispersion, topography, access, and orientation results. Values such as 200 m by 100 m for reconnaissance or 25 m by 25 m for follow-up are illustrations, not prescriptions.

3. Rock Chip & Trench Sampling

Rock sampling provides direct geochemical evidence of bedrock mineralization.

• Grab and Characterization Sampling: Grab or characterization samples provide selective reconnaissance evidence from outcrop, float, or veins but are not grade-representative unless a defensible sampling design establishes representativity.
• Continuous Channel Sampling & Trenching: Excavated trenches cut across structural strike. Continuous channel samples are cut with controlled width, depth, orientation, and interval across the exposure. Dimensions such as a 5–10 cm channel and 1 m interval are illustrative; geology, grain size, expected grade variability, safety, and the required support determine the actual protocol.

Pathfinder Elements & Dispersion Halos

Hydrothermal ore-forming processes create primary zoning (in host rocks surrounding the orebody) and secondary zoning (in weathered regolith). Pathfinder elements are mobile, easily detectable trace elements associated with specific ore deposits that form wider dispersion halos than the target ore metals.

1. Epithermal Gold-Silver Systems

Epithermal deposits form at shallow depths ($< 1\text{ km}$) and low temperatures ($100\text{--}300^\circ\text{C}$). High volatile element mobilities produce broad trace-element halos:

• Primary Pathfinders: Arsenic ($As$), Antimony ($Sb$), Mercury ($Hg$), Tellurium ($Te$), Bismuth ($Bi$), Thallium ($Tl$).
• Zoning Vectoring: In high-sulfidation epithermal systems (e.g., Lepanto, Benguet), elevated $Hg$ and $Sb$ occupy shallow steam-heated clay caps, while $As$ and $Te$ envelope the underlying high-grade $Au\text{--}Cu\text{--}Ag$ silica-enargite ore zone.

2. Porphyry Copper-Gold-Molybdenum Systems

Porphyry systems represent large-volume, low-grade magmatic-hydrothermal systems with distinct concentric alteration and geochemical halos:

• Core Metal Suite: Copper ($Cu$), Molybdenum ($Mo$), Gold ($Au$).
• Peripheral Pathfinder Halo: Zinc ($Zn$), Lead ($Pb$), Manganese ($Mn$), Selenium ($Se$). Zinc, lead, manganese, and related element ratios can form broad peripheral patterns, sometimes at kilometre scale, but lithology, weathering, analytical support, and local deposit orientation must be established before using them as vectors.

3. Philippine Tropical Weathering Considerations

In tropical, high-rainfall environments like the Philippines, intense chemical weathering (lateritization) leaches mobile alkali and alkaline earth cations ($Mg^{2+}, Ca^{2+}, Na^+, K^+$), concentrating immobile elements ($Fe^{3+}, Al^{3+}, Ni^{2+}, Cr^{3+}$) in residual ferruginous limonite and saprolite horizons. Geochemical interpretations must differentiate hydromorphic dispersion (groundwater transport) from mechanical residual accumulation.

Remote Sensing & Spectral Geology

Remote sensing exploits optical satellite imagery and hyperspectral sensors to map lithology, structural lineaments, and hydrothermal alteration zones over inaccessible terrain.

1. Multispectral Platforms & Spectral Regions

Satellites measure reflected solar radiation across distinct wavelength regions:

• Visible and Near-Infrared (VNIR, $0.4\text{--}1.0\text{ }\mu\text{m}$): Detects electronic absorption transitions in transition metals, notably ferric ($Fe^{3+}$) and ferrous ($Fe^{2+}$) iron oxides.
• Shortwave Infrared (SWIR, $1.0\text{--}2.5\text{ }\mu\text{m}$): Measures vibrational absorption of hydroxyl ($OH^-$), carbonate ($CO_3^{2-}$), and sulfate ($SO_4^{2-}$) molecular bonds. SWIR is critical for identifying clay minerals, micas, and sulfates.
• Key spaceborne sources: archival ASTER, Sentinel-2, and Landsat-8/9. ASTER SWIR stopped acquiring usable new data in 2008, so its pre-failure archive can support alteration mapping but is not current imagery.

2. Band Ratios & Alteration Mineral Mapping

Band ratioing divides digital number (DN) values of one spectral band by another to highlight specific mineral absorption and reflectance features while reducing some illumination and albedo effects when ratios are validated:

• Iron Oxides (Gossans / Cappings): Ferric oxides and hydroxides produce diagnostic color and VNIR absorption contrasts. Validated ratios such as ASTER Band 2/Band 1 or Landsat-8/9 Band 4/Band 2 can enhance iron-rich surfaces, but vegetation, soil, illumination, atmospheric correction, sensor scaling, and non-ore ferric material make the result non-unique.
• Phyllic & Argillic Alteration: Clay minerals (kaolinite, alunite, illite, montmorillonite, white mica/sericite) display intense absorption features around $2.17\text{--}2.20\text{ }\mu\text{m}$ (ASTER Band 6). The ratio $(\text{Band } 4 + \text{Band } 7) / \text{Band } 6$ highlights phyllic (sericitic) and advanced argillic zones characteristic of porphyry and epithermal deposit cores.
• Propylitic Alteration: Chlorite, epidote, and calcite absorb at $2.30\text{--}2.35\text{ }\mu\text{m}$ (ASTER Band 8), mapped via $(\text{Band } 5 + \text{Band } 9) / \text{Band } 8$.

Exploration Geochemistry & Remote Sensing Method Summary