SUBAQUEOUS HOT SPRING Au-Ag

SUBAQUEOUS HOT SPRING Au-Ag
by Dani J. Alldrick
British Columbia Geological Survey

Ref: Sulfeto maciço epitermal, subaquoso, hidrotermal, veio, substituição, camada, vulcânicas

IDENTIFICATION SYNONYMS: Epithermal massive sulphide; subaqueous-hydrothermal deposits; Eskay- type deposit; Osorezan-type deposit. COMMODITIES (BYPRODUCTS): Ag, Au (Cu, Pb, Zn, As, Sb, Hg). EXAMPLES (British Columbia - Canada/International): Eskay Creek (104B008), Lulu (104B376); Osorezan, Vulcano Islands and Jade hydrothermal field (Japan), Mendeleev Volcano (Kurile Islands, Russia), Rabaul (Papua New Guinea), White Island (New Zealand), Bacon-Manito and Surigao del Norte (Phillippines). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Vein, replacement and synsedimentary bedded sulphides are deposited in volcanic rocks and associated sediments in areas of shallow lacustrine, fluvial or marine waters or in glacial subfloors. TECTONIC SETTING: Active volcanic arcs (both oceanic island arcs and continental margin arcs) are likely setting. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: 1) Water-filled reservoirs in active continental volcanic areas (crater lakes, playa lakes, stream flood plains, glacier subfloors). 2) Sea-flooded, breached calderas, or unconsolidated shallow marine sediments at the foot of a volcano. AGE OF MINERALIZATION: Presumably any age, oldest known example is Jurassic. HOST/ASSOCIATED ROCK TYPES: Mineralization hosted by intermediate to felsic flows and tuffs and minor intercalated sedimentary rocks. Pillow lavas, coarse epiclastic debris flows, and assorted subvolcanic feeder dikes are all part of the local stratigraphic package. DEPOSIT FORM: Highly variable. Footwall stockwork or stringer-style vein networks. Large, textureless massive sulphide pods, finely laminated stratiform sulphide layers and lenses, reworked clastic sulphide sedimentary beds, and epithermal-style breccia veins with large vugs, coarse sulphides and chalcedonic silica. All types may coexist in a single deposit. TEXTURE/STRUCTURE: Range from fine clastic sulphides and "framboid"-like chemical precipitates to very coarse grained sulphide aggregates in breccia veins. Structural styles include: vein stockworks, major breccia veins, stratabound and stratiform sulphide lenses and layers. ORE MINERALOGY (Principal and subordinate): Sphalerite, tetrahedrite, boulangerite, bournonite, native gold, native silver, amalgam, galena, chalcopyrite, enargite, pyrite, stibnite, realgar, arsenopyrite orpiment; metallic arsenic, Hg-wurtzite, cinnabar, aktashite, unnamed Ag-Pb-As-S minerals, jordanite, wurtzite, krennerite, coloradoite, marcasite, magnetite, scorodite, jarosite, limonite, anglesite, native sulphur. GANGUE MINERALOGY (Principal and subordinate): Magnesian chlorite, muscovite (sericite), chalcedonic silica, amorphous silica, calcite, dolomite, pyrobitumen, gypsum, barite, potassium feldspar, alunite with minor carbon, graphite, halite and cristobalite. ALTERATION MINERALOGY: Massive chlorite (clinochlore)-illite-quartz-gypsum-barite rock or quartz-muscovite-pyrite rock are associated with the near-footwall stockwork zones. Chlorite and pyrite alteration is associated with the deep-footwall stockwork zones where alteration minerals are restricted to fractures. Stratabound mineralization is accompanied by magnesian chlorite, muscovite, chalcedonic silica, calcite, dolomite and pyrobitumen. At the Osorezan hot spring deposits, pervasive silica and alunite microveinlets are the dominant alteration phases. GENETIC MODEL: Deposits are formed by "hot spring" (i.e.: epithermal) fluids vented into a shallow water environment. Fluids are magmatic in character, rather than meteoric. This concept contrasts with some characteristics of the process model for volcanogenic massive sulphides. Lateral and vertical zoning has been recognized within a single lens. Lateral zoning shows changes from Sb, As and Hg-rich mineral suites to Zn, Pb and Cu-rich assemblages. Vertical zoning is expressed as a systematic increase in Au, Ag and base metal content up-section. Fluid conduits are fissures generated by seismic shock, aggradation of the volcano over a later expanding magma chamber, or fracturing in response to regional compressional tectonics. A near-surface subvolcanic magma body is an essential source of metals, fluids and heat. ASSOCIATED DEPOSIT TYPES: Hot spring Hg , hot spring Au-Ag , epithermal veins , volcanogenic exhalative massive sulphides . COMMENTS: This deposit type is the shallow subaqueous analogue of hot spring Au- Ag, and both of these are subtypes of the "epithermal" class of mineral deposits. Considering the recent discoveries at Osorezan (1987) and Eskay Creek (1988), the brief discussion by Laznicka (1985, p. 907) seems especially prophetic. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Ag, Au, Cu, Pb, Zn, As, Sb, Hg. GEOPHYSICAL SIGNATURE: The pyrite associated with stockwork mineralization and ubiquitous alteration should produce a widespread induced polarization anomaly, but the best targets may be local peaks within this broad anomalous 'plateau'. Airborne magnetometer surveys may help delineate favourable strata and fault offsets. OTHER EXPLORATION GUIDES: The geological deposit model and its regional setting may be the best exploration tools available. Broad hydrothermal systems marked by widespread sericite-pyrite alteration; evidence of a volcanic crater or caldera setting; accumulations of felsic volcanic strata: 1) in a local subaqueous setting in a regionally subaerial environment, 2) along the near shore zone of a regional subaerial/subaqueous volcanic facies transition (e.g.: the western margin of the Hazelton trough). Focus on the sedimentary intervals within the volcanic pile. ECONOMIC FACTORS GRADE AND TONNAGE: These deposits are not well known. The Eskay Creek deposit is attractive because of the polymetallic signature and high precious metal contents. It contains an estimated mining reserve of 1.08 Mt grading 65.5 g/t Au, 2930 g/t Ag, 5.7 % Zn, 0.77 % Cu and 2.89% Pb with geological reserves of 4.3 Mt grading 28.8 g/t Au and 1 027 g/t Ag. IMPORTANCE: These deposits are attractive because of their bonanza grades and polymetallic nature.