AG 3103 02

Mineral deposits are formed through the precipitation of ore constituents from hot, aqueous fluids (hydrothermal fluids) of variable composition and origin.
Hydrothermal solutions contribute to many types of deposits including veins, stockworks, and volcanic-exhalative deposits.
These solutions can carry a variety of materials, capable of depositing minerals like gold and muscovite, indicating complex physical chemistry difficult to replicate in laboratory settings.

Four sources of subsurface hydrothermal waters have been identified:
- Meteoric: Surface and groundwater, referred to as meteoric water.
- Connate: Formation water trapped in sediment pore spaces.
- Metamorphic: Water produced by metamorphic dehydration reactions or equilibrated with metamorphic rocks above 300°C.
- Magmatic: Water derived from magma.
Initially, most formation water is meteoric, but reactions with rock minerals during long burial alter its characteristics.

Hydrothermal deposits can be classified based on the temperature and depth of formation:
- Hypothermal: Formed at greater depths (300-500°C).
- Mesothermal: Formed at intermediate depths (200-300°C).
- Epithermal: Formed at shallow depths (50-200°C).
Additionally, deposits are categorized into:
- Telethermal: Low temperature and pressure.
- Xenothermal: High temperature at shallow depths.

Hydrothermal fluids range from moderately hot (50-500°C) aqueous solutions with various dissolved constituents, including gases.
Fluid salinity can vary widely (less than 5% to over 40%), containing chlorine, sodium, calcium, magnesium, potassium, heavy metals (Fe, Mn, Cu, Zn, Pb), sulfur, carbon, and nitrogen.
The pH typically ranges from weakly alkaline to somewhat acidic.

Significant deposits formed through magmatic hydrothermal processes include porphyry deposits and volcanic-associated sulfide deposits, crucial suppliers of copper and molybdenum.
Porphyry deposits are increasingly recognized as important gold sources, while volcanic-associated deposits provide substantial quantities of zinc and lead.

Characterized as large, rare ore bodies containing copper, molybdenum, gold, silver, and other metals formed from hydrothermal fluids originating in magma chambers.
Named for the porphyritic intrusive rocks associated with the fluids, often found in granitic or dioritic forms with copper minerals like chalcopyrite.

Most deposits formed in the last 75 million years, with few identified older than 450 million years.
Typically consist of large-tonnage, low-grade deposits suitable for bulk mining; mineralization includes disseminated formations of pyrite, chalcopyrite, bornite, and molybdenite.
Secondary enrichment occurs with supergene blankets leading to minerals like chalcocite and djurleite.

Usually associated with small porphyritic intrusions, commonly acid intrusives from granite suites.
Difficult to ascertain primary rock compositions due to alteration, often containing diorites and other igneous rocks compatible with the deposit's formation environment.

Characterized by pervasive and selective alteration phenomena, often enhancing exploration targets.
Alteration types include:
- Potassic Zone: Biotite, orthoclase, quartz.
- Phyllic Zone: Sericitic alteration leading to chalcopyrite presence.
- Argillic Zone: Clay minerals, indicating extensive alteration.
- Propylitic Zone: Chlorite and epidote formation.

Found commonly at convergent plate margins characterized by calcalkaline magmatism and oceanic crust subduction, generating hydrous granitoids linked to ore formation.

Result from hydrous magmas intruding into permeable cover rocks at shallow depths, producing heat and energy for ore deposition through convective fluid flow.
Evidence suggests fluids originate from crystallizing magmas, with subsequent involvement from meteoric and connate waters.

Formed in volcanic environments, characterized by stratiform sulfide accumulations precipitated from hydrothermal fluids on or beneath the seafloor.
Typically polymetallic, containing combinations of Cu, Zn, Pb, Au, and Ag, with various other metals as by-products.

These epigenetic Pb-Zn sulfide deposits occur in indurated carbonate rocks and are often driven by faulting and topography.
Characterized by sodium-calcium-chloride brines, significant at low temperatures (60-160°C) and higher salinities than seawater.