Here are the exploration implications distilled from Sillitoe & Hedenquist (2025) on alkalic porphyry deposits and advanced argillic lithocaps:
Key Implications for Exploration
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Lithocaps can form above alkalic porphyries
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Alkaline volcanic systems emit SO₂, HCl, and HF in amounts comparable to calc-alkaline arcs, so they are capable of producing the hyperacidic condensates that generate lithocaps.
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The scarcity of preserved lithocaps in alkalic porphyry settings is due mainly to erosional removal, not genetic absence2025_Sillitoe and Hedenquist - …2025_Sillitoe and Hedenquist - ….
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Preservation potential depends on age and erosion
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Lithocaps are observed only in Cenozoic alkalic porphyries (e.g., Kışladağ, Didipio, Nasivi 3, Fakos).
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Paleozoic and Mesozoic examples (Cadia, Northparkes, British Columbia deposits) lack them because >1 km of cover has been eroded2025_Sillitoe and Hedenquist - ….
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Thus, lithocap preservation is a strong function of tectonic setting, uplift, and erosional history.
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Exploration vectors from lithocap remnants
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Where preserved, lithocaps are commonly barren or weakly mineralized, but indicate the presence of a porphyry system at depth.
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Recognizing structurally controlled root zones (white mica ± kaolinite ± carbonate alteration) is important—they may represent the eroded feeder zones of lithocaps and point toward porphyry mineralization below.
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Submarine or high-permeability volcanic settings may limit lithocap extent
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In submarine settings (e.g., Cadia), lithocaps may have been smaller or modified by seawater interaction.
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High permeability in volcanic edifices (e.g., Etna-type volcanoes) can flush acidic condensates, reducing lithocap development2025_Sillitoe and Hedenquist - ….
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Carbonate buffering reduces lithocap expression
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Alkalic porphyry alteration often contains more carbonate, which can buffer fluids to higher pH and inhibit advanced argillic overprinting. This may result in weaker or patchier lithocap development compared to calc-alkaline systems2025_Sillitoe and Hedenquist - ….
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Exploration strategy implications
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Do not assume absence of lithocap = absence of alkalic porphyry system. Older deposits may have lost their lithocaps entirely.
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In preserved systems, lithocaps (quartz-alunite ± pyrophyllite ± diaspore) provide direct evidence for shallow-level intrusions and potential porphyry Cu–Au mineralization beneath.
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Given that many alkalic porphyries are related to very small intrusions (“pencil porphyries”), drilling beneath lithocaps can be challenging but necessary2025_Sillitoe and Hedenquist - ….
✅ Overall takeaway for explorers:
Advanced argillic lithocaps can form over alkalic porphyry deposits, but their preservation depends strongly on age and erosion. Lithocap remnants—when present—are valuable guides to concealed porphyries. In older terrains (e.g., British Columbia, NSW), porphyry exploration should not rely on lithocap evidence, but instead focus on recognizing deeper alteration assemblages and structural roots.