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Ministry of Energy Mines and Responsible for Core Review

H - Epithermal

BC Profile # Deposit Type Approximate Synonyms USGS Model #
H01 Travertine Tufa 35d*
H02 Hot spring Hg - - 27a
H03 Hot spring Au-Ag - - 25a
H04 Epithermal Au-Ag-Cu; high sulphidation Acid-sulphate, qtz-alunite Au, Nansatsu-type 25d
H05 Epithermal Au-Ag; low sulphidation Adularia-sericite epithermal 25c
  H06* Epithermal Mn - - 25g
H07 Sn-Ag veins Polymetallic Sn veins 25h, 20b
H08 Alkalic intrusion-associated Au Alkalic intrusion-related Au, Au-Ag-Te veins 22b
H09 Hydrothermal alteration clays-Al-Si Kaolin, Alunite, Siliceous cap, Pyrophyllite 25lb*

TRAVERTINE

H01
by Z.D. Hora
British Columbia Geological Survey

 

Hora, Z.D. (1996): Travertine, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 29-30.

 

IDENTIFICATION

 

SYNONYMS: Tufa, calcareous sinter; certain varieties also referred to as onyx marble or Mexican onyx.

 

COMMODITIES (BYPRODUCTS): Decorative stone, building stone products, soil conditioner, agriculture lime; onyx marble.

 

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Clinton (092P 079), Slocan (082KSW074, 075), Wishing Well (Deep River, 094N 001); Gardiner (Montana, USA), Salida (Colorado, USA), Bridgeport (California, USA); Lazio, Tuscany (Italy); Pamukkale (Turkey); Mexico, Spain, Iran.

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Mounds, sheets, sometimes terraced, shallow lake in-fills, valley in-fill.

 

TECTONIC SETTING: Young orogenic belts with carbonate sediments in the subsurface; thrusts and faults with deep water circulation. Also intercontinental rift zones with strike-slip faulting, with or without associated volcanic activity.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Subaerial precipitation of calcium carbonate from mineral springs; also in shallow lacustrine basins with influx of mineralized CO2-rich water. Hotspring waters which give rise to travertine deposits usually do not originate at temperatures in excess of 100oC. Circulating ground waters are channeled by thrusts, faults and fractured rocks and mineralized by dissolution of subsurface carbonate rocks.

 

AGE OF MINERALIZATION: Tertiary to recent.

 

HOST/ASSOCIATED ROCK TYPES: Carbonate rocks in the subsurface; hydrothermal breccia and siliceous sinters, lacustrine sediments, carbonate veins (usually aragonite) in form of “Mexican onyx”.

 

DEPOSIT FORM: Conical mounds, sheets, basin in-fills. As it is deposited by precipitation from warm spring waters, it shows successive layers with sometimes different colours and textures. May be elongated above underlying feeder zones following faults and breccia zones.

 

TEXTURE: Banded, porous, brecciated; may be pisolitic. Generally fine-grained carbonate matrix with numerous irregular cavities ranging in size from a pin head to 1 cm or more across. The cavities are usually oriented in lines giving the rock parallel texture. Lacustrine varieties are more massive. The mounds may be criss-crossed by veins of “Mexican onyx”, a varicoloured banded aragonite.

 

ORE MINERALOGY (Principal and subordinate): Calcite, aragonite, silica, fluorspar, barite, native sulphur.

 

WEATHERING: Clay/iron stains filling the voids, joints and bedding planes.

 

ORE CONTROLS: Commonly developed along high-angle faults and shear zones in young orogenic belts.

 

GENETIC MODEL: Travertine forms as surface deposits from geothermal systems of generally less than 100oC in temperature. The carbonate deposition results from the loss of some of the carbon dioxide by cooling, evaporation or presence of algae.

 

ASSOCIATED DEPOSIT TYPES: Hotsprings Au-Ag (H03), Hotspring Hg (H02), marl, solfatara sulphur, geyserite silica.

 

COMMENTS: To be economically of interest, the size must be suitable to open a quarry face, the carbonate must be recrystallized and cemented to be strong and hard for ornamental stone applications. Sediments of similar texture and composition may occur in karst regions, where the carbonate precipitated from cold water.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Mineral springs with carbon dioxide.

 

OTHER EXPLORATION GUIDES: Precipitation of tufa from small streams on moss and other organic matter, presence of thermal spring and solfatara exhalations.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Large deposits may reach 1-2 Mt, but even the small deposits of several tens to a hundred thousand tonnes may be of importance for local and custom type work. The travertine has to meet the minimum physical test requirements for intended use.

 

END USES: Interior and exterior facing, tile, ashlar, custom-made shapes as steps and sills, lapidary work and precious stone applications.

 

ECONOMIC LIMITATIONS: Even small occurrences can be exploited for local and custom markets.

 

IMPORTANCE: Locally important facing stone, however the usage does not match marble or granite. Mexican onyx is an important decorative stone.

 

REFERENCES

 

Carr, D.D. (1994): Industrial Minerals and Rocks; Society for Mining, Metallurgy, and Exploration, Littleton, Colorado, 1196 pages.

Harben, P.W. and Bates, R.L. (1990): Industrial Minerals Geology and World Deposits; Metal Bulletin, London, 135 pages.

Kuzvart, M. (1984): Deposits of Industrial Minerals; Academia, Prague, 440 pages.

Robbins, J. (198 ): Italy’s Industrial Minerals; Industrial Minerals, No. 255, pages 19- 45.

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HOT SPRING Hg


H02

by A. Panteleyev
British Columbia Geological Survey

 

Panteleyev, A. (1996): Hot-spring Hg, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 31-32.

 

IDENTIFICATION

 

SYNONYMS: (Epithermal) hotspring, subaerial siliceous sinter.

 

COMMODITIES (BYPRODUCTS): Hg, (Au).

 

EXAMPLES (British Columbia - Canada/International): Ucluelet; Knoxville district, Sulphur Bank (California, USA), McDermitt and Steamboat Springs (Nevada, USA), Abuta mine(Japan).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Uppermost portions of epithermal systems develop clay altered zones and siliceous caps a few metres to hundreds of metres below surface and silica sinter deposits above the groundwater table as hotspring deposits. Travertine ledges and other silica-carbonate accumulations may be present nearby as peripheral or deeper deposits.

 

TECTONIC SETTING: Continental margin rifting and strike-slip faulting associated with small volume mafic to intermediate volcanism.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Modern and fossil hotspring settings with silica and silica-carbonate deposition near the paleo groundwater table and as subaerial silica sinter precipitates.

 

AGE OF MINERALIZATION: Tertiary and younger; some currently active hotsprings.

 

HOST/ASSOCIATED ROCK TYPES: Intermediate to basic volcanic flows, tuffs and breccias, minor diabasic dykes; hydrothermal breccias, travertine and siliceous sinters, lacustrine sediments. Country rocks commonly include greywacke, shale and fault-related serpentinized ultramafic bodies.

 

DEPOSIT FORM: Lensoid hotspring deposits and tabular lithologic replacement zones; commonly with cone- or wedge-like underlying feeder zones centered on regional- scale fault and fracture zones. Commonly less than 300 metres in vertical extent from paleosurface. Locally phreatic explosion pits.

 

TEXTURE: Disseminated sulphides in country rocks and hydrothermal breccias, quartz stockworks of banded to vuggy, multiple-generation quartz-chalcedony veins. Hydrofracturing textures are common. Less frequently cinnabar occurs as grains, lenses and fracture coatings in opaline silica sinter deposits. In some deposits cinnabar is concentrated on surfaces of wood and other organic matter.

 

ORE MINERALOGY (Principal and subordinate): Cinnabar, pyrite, native sulphur and mercury, stibnite, gold, marcasite.

 

GANGUE MINERALOGY (Principal and subordinate): Quartz, chalcedony; opal, carbonate, iron oxides, manganese oxides.

 

ALTERATION MINERALOGY (Principal and subordinate): Kaolinite, alunite, Fe-Mn oxides and sulphur above water table (minor amounts of cinnabar). Opaline quartz deposited at the water table, with cinnabar. Quartz, pyrite, zeolites, chlorite and minor adularia below the water table; silica-carbonate ñ magnesite assemblages in mafic, commonly serpentinized, rocks.

 

GENETIC MODEL: Deposits form in geothermal systems from near surface hot waters at less than 150§C, and generally cooler. Organic materials in solution and high CO2 vapour concentration may be important in the transporting of elevated amounts of Hg.

 

ORE CONTROLS: Located just below the paleo groundwater table within hotspring systems. Commonly developed along high-angle faults and generally in young volcanic terranes.

 

ASSOCIATED DEPOSIT TYPES: Hotspring Au-Ag (H03), epithermal Au-Ag (H04, H05), placer Au (C01, C02).

 

COMMENTS: There has been little work in recent years on this deposit type other than to examine their potential for related gold deposits, for example, McLaughlin mine in California (Gustafson, 1991). The significant Hg deposits typically contain no other recoverable constituents.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Hg, Sb, As. Generally <5 ppb Au but rare deposits with elevated gold are known.

 

GEOPHYSICAL SIGNATURE: VLF to identify favourable structures; magnetic lows in mafic volcanic hosts due to alteration envelope.

 

OTHER EXPLORATION GUIDES: Can be overlain by native sulphur occurrences or hot spring deposits with siliceous sinters and clay-altered rocks. Recent deposits are commonly associated with modern hot springs or geothermal fields. Silica- carbonate alteration with distinctive orange-coloured, amorphous limonite in weathered zones, typically in mafic and serpentizized hostrocks.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Commercially exploited deposits tend to be very small; the largest deposits rarely exceed 1 mt in size. The median production from 20 Cordilleran USA mines is <1000 tonnes with 0.35% Hg. Typical mineable reserves contain ores ranging from 0.2 to 0.6% Hg. Productive deposits are Sulphur Bank and 5 small mines in the Knoxville District in California which produced 4, 700 tonnes of Hg and ~5,520 tonnes Hg respectively.

 

ECONOMIC LIMITATIONS: There probably is no operating mine of this type in the world today.

 

IMPORTANCE: These are relatively small deposits from near surface geological environments that are easily eroded and therefore rarely preserved. They currently are not important sources of mercury but can be associated with auriferous epithermal deposits.

 

REFERENCES

 

Berger, B. (1985): Geologic-Geochemical features of hot-spring precious metal deposits: U.S. Geological Survey, Bulletin 1646, pages 47-53.

Gustafson, D.L. (1991): Anatomy of a Discovery: The McLaughlin Gold Mine, Napa, Yolo, and Lake Counties, California: Economic Geology, Monograph 8, pages 350-359.

Rytuba, J.J. (1986): Descriptive Model of Hot-spring Hg; in Mineral Deposit Models, U.S. Geological Survey, Bulletin 1693, pages 178, 179.

Sherlock, R.L. and Logan, M.A.V. (in press): Silica-Carbonate Alteration of Serpentinite: Implications for the Association of Mercury and Gold Mineralization in Northern California; Exploration and Minining Geology, Canadian Institute of Mining and Metallurgy, in press.

White, D. (1981): Active Geothermal Systems and Hydrothermal Ore Deposits; Economic Geology, 75th Anniversary Volume, pages 392-423.

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HOT-SPRING Au-Ag


H03
by A. Panteleyev
British Columbia Geological Survey

 

Panteleyev, A.(1996): Hot-spring Au-Ag, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy,T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 33-36.

 

IDENTIFICATION

 

SYNONYMS: (Epithermal) hotspring, subaerial siliceous sinter.

COMMODITIES (BYPRODUCTS): Au, (Ag, Hg).

 

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Cinola (uppermost part, 103F 034), Clisbako (093C 016), Wolf? (093F 045), Trout? (093F 044); McLaughlin (California, USA), Round Mountain (Nevada, USA).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Auriferous chalcedonic or opaline silica and fine-grained quartz form veins, stockworks and matrix filling in breccias hosted by volcanic and, less commonly, sedimentary rocks. These are the uppermost parts of epithermal systems which develop mineralized siliceous caps a few metres to hundreds of metres below surface with subaerial siliceous sinter deposits at the water table and explosion breccias above.

 

TECTONIC SETTINGS: Continental margin rifting and district-scale fracture systems with associated bimodal or low volume mafic to intermediate volcanism. Commonly in regions of strike-slip faulting with transform faults and transtensional basin margins. Also extensional tectonism with related caldera development and resurgence, flow-dome complexes and high-level subvolcanic intrusive activity.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Shallow parts of fossil geothermal systems. Hotsprings deposit silica near the paleo groundwater table and as subaerial, ponded precipitates. Deeper fluids are channeled by permeable stratigraphic units, hydrothermal breccia bodies and faulted/fractured rocks. Subaerial volcanic centres including flow-dome or caldera complexes and related radial and ring fracture systems.

 

AGE OF MINERALIZATION: Tertiary and Quaternary are most common; some currently active hotsprings. Hotspring sinters as old as Late Devonian have been described (Cunneen and Sillitoe, 1989).

 

HOST/ASSOCIATED ROCK TYPES: Intermediate or bimodal basaltic-rhyolitic volcanics including volcanic flows, flow domes, tuffs and breccias; hydrothermal breccias and siliceous sinters. Any type of permeable or structurally prepared country rock can be mineralized, most commonly ash flow units and caldera-fill sediments. In some cases, serpentinized ultramafic and mafic rocks in major fault zones in areas of post-faulting volcanic activity are mineralized. Sedimentary rocks occur at Cinola and many other deposits.

 

DEPOSIT FORM: Near-surface, lensoid hotspring deposits and planar lithologic replacement zones. Individual zones are up to hundreds of metres in two dimensions and tens of metres in the third. Underlying these are cone or wedge-like hydrothermal feeder systems with quartz stockworks and veins centred on regional-scale fault and fracture zones, or their splays. Locally phreatic and phreatomagmatic explosion pits formed at the paleosurface.

 

TEXTURE/STRUCTURE: Generally very fine grained disseminated sulphides in silicified (opalized and chalcedonic) country rocks and silica sinter; hydrothermal breccias, quartz stockworks and banded to vuggy, sheeted, multiple-generation quartz- chalcedony veins. Hydrofracturing textures are common.

 

ORE MINERALOGY (Principal and subordinate): Pyrite, marcasite, gold, electrum; stibnite, sulphosalt minerals, realgar, cinnabar (cinnibar only near tops of deposits).

 

GANGUE MINERALOGY (Principal and subordinate): Quartz, chalcedony; opal, calcite, dolomite, barite. Strong silicification with quartz, chalcedony and opal in crustified, banded veins, sheeted veins and stockworks is characteristic in ores. Silica in some deposits contains abundant hydrocarbons that impart a characteristic brownish colour to the quartz.

 

ALTERATION MINERALOGY (Principal and subordinate): Multiple episodes of silicification to form veins and stockworks, and pervasive silicified hostrocks adjacent to them, is typical. Country rocks containing the silicified zones have argillic and, less commonly, advanced argillic assemblages with quartz-kaolinite and rarely alunite. They are flanked, or underlain, by propylitic rocks with chlorite, Fe oxides, zeolites and minor adularia. Selenite, alunite and other sulphate minerals and native sulphur can be abundant locally near surface.

 

WEATHERING: Limonite (jarosite, hematite, goethite) is locally prominent near surface in strongly oxidized deposits.

 

ORE CONTROLS: A key element at the McLaughlin deposit was the superposition of multiple generations of auriferous veinlets each carrying a small amount of gold (Lehrman, 1986).

 

GENETIC MODEL: Hydrothermal breccias and multiple generations of veins with calcite replacement by silica attest to boiling of hydrothermal fluids as an important ore-depositing mechanism. The boiling levels are related to the paleosurface and commonly have a surficial expression as active or paleo-hotsprings. The deeper hydrothermal fluid systems, generaly within 500 m of surface (paleosurface for older deposits), can be developed along active, regional high-angle faults and other volcanic and subvolcanic intrusion-related structures. The structures commonly cut or flank domes in flow-dome complexes.

 

ASSOCIATED DEPOSIT TYPES: Hotspring Hg (H02), solfatara sulphur; epithermal Au-Ag (H04, H05), placer Au (C01, C02).

 

COMMENTS: Many deposits currently being exploited throughout the world have grades between 1 and 2 g/t Au and range from a few to tens of millions of tonnes in size. They are viable generally because the rocks are commonly strongly oxidized and the gold can be recovered by heap leaching methods. The siliceous sinters formed at or very near to the surface rarely contain economic mineralization These deposits have a greater depth extent then hotspring mercury deposits. In their deeper parts they may grade into precious metal bearing and base metal epithermal veins.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Au, Sb, As, Hg, Tl near surface, increasing Ag, Ba at depth; locally Ni, B, Li and W. The Ag/Au ratio varies from 1:1 at surface to 30:1 at a depth of a few hundred metres. Mineralized rocks can be strongly leached at surface. Notably absent are: Se, Te, F, Mo, Sn and Mn. Base metal content is relatively low, for example, common amounts are Cu <60 ppm, Pb <5 ppm and Zn <450 ppm.

 

GEOPHYSICAL SIGNATURE: Resistivity, VLF to identify faults.

 

OTHER EXPLORATION GUIDES: Siliceous sinter can be used to identify the paleosurface; Hg mineralization may overlie deeper gold ores.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Mineralization tends to be low grade. Economically attractive bulk-mineable deposits contain >10 Mt of 1 to 2 g/t Au, or greater. High-grade veins and stockworks within the larger mineralized zones can be exploited by underground methods. The McLaughlin deposit, a superior discovery, contained initial reserves of 17.5 Mt with 5.2 g/t Au and about 16 g/t Ag, including a sheeted vein zone with 2.45 Mt with 9.15 g/t Au. Reserves for Cinola are about 31 Mt with 2.19 g/t Au; the deposit has a feeder zone at depth that contains material containing in excess of 100 g/t Au.

 

ECONOMIC LIMITATIONS: Refractory primary ore in deposits that lack significant oxidation renders many of the lower grade deposits uneconomic.

 

IMPORTANCE: Individual deposits are attractive economically, for example, the McLaughlin mine in California.

 

REFERENCES

 

Berger, B.R. (1985): Geologic-Geochemical Features of Hot-spring Precious Metal Deposits: U.S. Geological Survey, Bulletin 1646, pages 47-53.

Berger, B.R. (1986): Descriptive Model of Hot-spring Au-Ag; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors,U.S. Geological Survey, Bulletin 1693, page 143.

Cunneen, R. and Sillitoe, R.H. (1989): Paleozoic Hot Spring Sinter in the Drummond Basin, Queensland, Australia; Economic Geology, Volume 84, pages 135-142.

Lehrman, N.J. (1986): The McLaughlin Mine, Napa and Yolo Counties, California; Nevada Bureau of Mines and Geology, Report 41, pages 85-89.

Peters, E.K. (1991): Gold-bearing Hot Spring Systems of the Northern Coast Ranges, California; Economic Geology, Volume 86, pages 1519-1528.

Sillitoe, R.H. (1993): Epithermal Models: Genetic Types, Geometric Controls and Shallow Features; in Ore Deposits Modeling, Geological Association of Canada, Special Volume 40, pages 403-417.

Tosdal, R.M., Enderlin, D.A., Nelson, G.C. and Lehrman, N.J. (1993): Overview of the McLaughlin Precious Metal Deposit, Napa and Yolo Counties, Northern California; in Active and Geothermal Systems and Gold/Mercury Deposits in the Sonoma/Clear Lake Volcanic Fields, California, J.J. Rytuba, Editor, Society of Economic Geologists, Guidebook Series, Volume 16, pages 312-329.

White, D. (1981): Active Geothermal Systems and Hydrothermal Ore Deposits; Economic Geology, 75th Anniversary Volume, pages 392-423.

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EPITHERMAL Au-Ag-Cu: HIGH SULPHIDATION


H04
by A. Panteleyev
British Columbia Geological Survey

 

Panteleyev, A. (1996): Epithermal Au-Ag-Cu: High Sulphidation, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 37-39.

 

IDENTIFICATION

 

SYNONYMS: (Epithermal) acid-sulphate, quartz-alunite Au, alunite-kaolinite ± pyrophyllite, advanced argillic, Nansatsu-type, enargite gold. The deposits are commonly referred to as acid-sulphate type after the chemistry of the hydrothermal fluids, quartz-alunite or kaolinite-alunite type after their alteration mineralogy, or high-sulphidation type in reference to the oxidation state of the acid fluids responsible for alteration and mineralization.

 

COMMODITIES (BYPRODUCTS): Au, Ag, Cu (As, Sb).

 

EXAMPLES (British Columbia (MINFILE #) - International): Mt. McIntosh/Hushamu (EXPO, 092L 240), Taseko River deposits - Westpine (Empress) (092O 033), Taylor-Windfall (092O 028) and Battlement Creek (092O 005); Goldfield and Paradise Peak (Nevada, USA), Summitville (Colorado, USA); Nansatsu (Japan), El Indio (Chile); Temora (New South Wales, Australia), Pueblo Viejo (Dominica), Chinkuashih (Taiwan), Rodalquilar (Spain), Lepanto and Nalesbitan (Philippines).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Veins, vuggy breccias and sulphide replacements ranging from pods to massive lenses occur in volcanic sequences associated with high level hydrothermal systems marked by acid-leached, advanced argillic, siliceous alteration.

 

TECTONIC SETTING: Extensional and transtensional settings, commonly in volcano-plutonic continent-margin and oceanic arcs and back-arcs. In zones with high-level magmatic emplacements where stratovolcanoes and other volcanic edifices are constructed above plutons.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Subvolcanic to volcanic in calderas, flow-dome complexes, rarely maars and other volcanic structures; often associated with subvolcanic stocks and dikes, breccias. Postulated to overlie, and be genetically related to, porphyry copper systems in deeper mineralized intrusions that undelie the stratovolcanoes.

 

AGE OF MINERALIZATION: Tertiary to Quaternary; less commonly Mesozoic and rarely Paleozoic volcanic belts. The rare preservation of older deposits reflects rapid rates of erosion before burial of subaerial volcanoes in tectonically active arcs.

 

HOST/ASSOCIATED ROCK TYPES: Volcanic pyroclastic and flow rocks, commonly subaerial andesite to dacite and rhyodacite, and their subvolcanic intrusive equivalents. Permeable sedimentary intervolcanic units can be sites of mineralization.

 

DEPOSIT FORM: Veins and massive sulphide replacement pods and lenses, stockworks and breccias. Commonly irregular deposit shapes are determined by hostrock permeability and the geometry of ore-controlling structures. Multiple, crosscutting composite veins are common.

 

TEXTURE/STRUCTURE: Vuggy 'slaggy' silica derived as a residual product of acid leaching is characteristic. Drusy cavities, banded veins, hydrothermal breccias, massive wallrock replacements with fine-grained quartz.

 

ORE MINERALOGY (Principal and subordinate): pyrite, enargite/luzonite, chalcocite, covellite, bornite, gold, electrum; chalcopyrite, sphalerite, tetrahedrite/tennantite, galena, marcasite, arsenopyrite, silver sulphosalts, tellurides including goldfieldite. Two types of ore are commonly present: massive enargite-pyrite and/or quartz-alunite-gold.

 

GANGUE MINERALOGY (Principal and subordinate): Pyrite and quartz predominate. Barite may also occur; carbonate minerals are absent.

 

ALTERATION MINERALOGY (Principal and subordinate): Quartz, kaolinite/dickite, alunite, barite, hematite; sericite/illite, amorphous clays and silica, pyrophyllite, andalusite, diaspore, corundum, tourmaline, dumortierite, topaz, zunyite, jarosite, Al-P sulphates (hinsdalite, woodhouseite, crandalite, etc.) and native sulphur. Advanced argillic alteration is characteristic and can be areally extensive and visually prominent. Quartz occurs as fine-grained replacements and, characteristically, as vuggy, residual silica in acid-leached rocks.

 

WEATHERING: Weathered rocks may contain abundant limonite (jarosite-goethite-hematite), generally in a groundmass of kaolinite and quartz. Fine-grained supergene alunite veins and nodules are common.

 

ORE CONTROLS: In volcanic edifices - caldera ring and radial fractures; fracture sets in resurgent domes and flow-dome complexes, hydrothermal breccia pipes and diatremes. Faults and breccias in and around intrusive centres. Permeable lithologies, in some cases with less permeable cappings of hydrothermally altered or other cap rocks. The deposits occur over considerable depths, ranging from high-temperature solfataras at paleosurface down into cupolas of intrusive bodies at depth.

 

GENETIC MODEL: Recent research, mainly in the southwest Pacific and Andes, has shown that these deposits form in subaerial volcanic complexes or composite island arc volcanoes above degassing magma chambers. The deposits can commonly be genetically related to high-level intrusions. Multiple stages of mineralization are common, presumably related to periodic tectonism with associated intrusive activity and magmatic hydrothermal fluid generation.

 

ASSOCIATED DEPOSIT TYPES: Porphyry Cu±Mo±Au deposits (L04), subvolcanic Cu-Ag-Au (As- Sb) (L01), epithermal Au-Ag deposits: low sulphidation type (H05), silica-clay-pyrophyllite deposits (Roseki deposits) (H09), hotspring Au-Ag (H03), placer Au deposits (C01, C02).

 

COMMENTS: High-sulphidation epithermal Au-Ag deposits are much less common in the Canadian Cordillera than low-sulphidation epithermal veins. However, they are the dominant type of epithermal deposit in the Andes.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Au, Cu, As dominate; also Ag, Zn, Pb, Sb, Mo, Bi, Sn, Te, W, B and Hg.

 

GEOPHYSICAL SIGNATURE: Magnetic lows in hydrothermally altered (acid-leached) rocks; gravity contrasts may mark boundaries of structural blocks.

 

OTHER EXPLORATION GUIDES: These deposits are found in second order structures adjacent to crustal-scale fault zones, both normal and strike-slip, as well as local structures associated with subvolcanic intrusions. The deposits tend to overlie and flank porphyry copper-gold deposits and underlie acid-leached siliceous, clay and alunite-bearing ‘lithocaps’.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: There is wide variation in deposit types ranging from bulk- mineable, low-grade to selectively mined, high-grade deposits. Underground mines range in size from 2 to 25 Mt with grades from 178 g/t Au, 109 g/t Ag and 3.87% Cu in direct smelting ores (El Indio) to 2.8 g/t Au and 11.3 g/t Ag and 1.8% Cu (Lepanto). Open pit mines with reserves of <100 Mt to >200 Mt range from Au-Ag mines with 3.8 g/t Au and 20 g/t Ag (Pueblo Viejo, Dominica) to orebodies such as the Nansatsu deposits, Japan that contain a few million tonnes ore grading between 3 and 6 g/t Au. Porphyry Au (Cu) deposits can be overprinted with late-stage acid sulphate alteration zones which can contain in the order of ~1.5 g/t Au with 0.05 to 0.1% Cu in stockworks (Marte and Lobo) or high-grade Cu-Ag-Au veins (La Grande veins, Collahausi). More typically these late stage alteration zones carry <0.4 to 0.9 g/t Au and >0.4 to 2% Cu (Butte, Montana; Dizon, Philippines).

 

ECONOMIC LIMITATIONS: Oxidation of primary ores is commonly neccessary for desireable metallurgy; primary ores may be refractory and can render low-grade mineralization noneconomic.

 

IMPORTANCE: This class of deposits has recently become a focus for exploration throughout the circum-Pacific region because of the very attractive Au and Cu grades in some deposits. Silica-rich gold ores (3-4 g/t Au) from the Nansatsu deposits in Japan are used as flux in copper smelters.

 

REFERENCES

 

Albino, G.V. (1994): Time-pH-fO2 Paths of Hydrothermal Fluids and the Origin of Quartz- Alunite-Gold Deposits; United States Geological Survey, Bulletin 2081, pages 33- 42.

Berger, B.R. (1986): Descriptive Model of Epithermal Quartz-Alunite Au; in Mineral Deposit Models, Cox, D.P. and Singer, D.A, Editors, U.S. Geological Survey, Bulletin 1693, page 158.

Henley, R.W. (1991): Epithermal Gold Deposits in Volcanic Terranes; in Gold Metallogeny and Exporation, R.P. Foster, Editor, Blackie and Sons Ltd, Glasgow, pages 133-164.

Mosier, D.L. and Menzie, W.D. (1986): Grade and Tonnage Model of Epithermal Quartz- Alunite Au, in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, page 158.

Panteleyev, A. (1991): Gold in the Canadian Cordillera - A Focus on Epithermal and Deeper Environments, in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 163-212.

Sillitoe, R.H. (1993): Epithermal Models: Genetic Types, Geometric Controls and Shallow Features; Geological Association of Canada, Special Volume 40, pages 403-417.

White, N.C. (1991): High Sulfidation Epithermal Gold Deposits: Characteristics and a Model for their Origin; in High-temperature Acid Fluids and Associated Alteration and Mineralization, Geological Survey of Japan, Report No. 277, pages 9-20.

White, N.C. and Hedenquist, J.W. (1990): Epithermal Environments and Styles of Mineralization: Variations and their Causes, and Guidelines for Exploration; in Epithermal Gold Mineralization of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II, Hedenquist, J.W. , White, N.C. and Siddeley, G., Editors, Journal of Exploration Geochemistry, Volume 36, pages 445-474.

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EPITHERMAL Au-Ag: LOW SULPHIDATION


H05
by A. Panteleyev
British Columbia Geological Survey

 

Panteleyev, A. (1996): Epithermal Au-Ag: Low Sulphidation, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 41-44.

 

IDENTIFICATION

 

SYNONYMS: (Epithermal) adularia-sericite; quartz-adularia, Comstock, Sado-type; bonanza Au-Ag; alkali chloride (hydrothermal).

 

COMMODITIES (BYPRODUCTS): Au, Ag (Pb, Zn, Cu).

 

EXAMPLES (British Columbia (MINFILE #) - International): Toodoggone district deposits - Lawyers (094E 066), Baker (094E 026), Shas (094E 050); Blackdome (092O 050, 092O 051, 092O 052, 092O 053); Premier Gold (Silbak Premier), (104B 054); Cinola (103F 034); Comstock, Aurora (Nevada, USA), Bodie (California, USA), Creede (Colorado, USA), Republic (Washington, USA), El Bronce (Chile), Guanajuato (Mexico), Sado, Hishikari (Japan), Colqui (Peru), Baguio (Philippines) Ladolam (Lihir, Papua- New Guinea).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Quartz veins, stockworks and breccias carrying gold, silver, electrum, argentite and pyrite with lesser and variable amounts of sphalerite, chalcopyrite, galena, rare tetrahedrite and sulphosalt minerals form in high- level (epizonal) to near-surface environments. The ore commonly exhibits open- space filling textures and is associated with volcanic-related hydrothermal to geothermal systems.

 

TECTONIC SETTING: Volcanic island and continent-margin magmatic arcs and continental volcanic fields with extensional structures.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: High-level hydrothermal systems from depths of ~1 km to surficial hotspring settings. Regional-scale fracture systems related to grabens, (resurgent) calderas, flow-dome complexes and rarely, maar diatremes. Extensional structures in volcanic fields (normal faults, fault splays, ladder veins and cymoid loops, etc.) are common; locally graben or caldera-fill clastic rocks are present. High-level (subvolcanic) stocks and/or dikes and pebble breccia diatremes occur in some areas. Locally resurgent or domal structures are related to underlying intrusive bodies.

 

AGE OF MINERALIZATION: Any age. Tertiary deposits are most abundant; in B.C. Jurassic deposits are important. Deposits of Paleozoic age are described in Australia. Closely related to the host volcanic rocks but invariably slightly younger in age (0.5 to 1 Ma, more or less).

 

HOST/ASSOCIATED ROCK TYPES: Most types of volcanic rocks; calcalkaline andesitic compositions predominate. Some deposits occur in areas with bimodal volcanism and extensive subaerial ashflow deposits. A less common association is with alkalic intrusive rocks and shoshonitic volcanics. Clastic and epiclastic sediments in intra-volcanic basins and structural depressions.

 

DEPOSIT FORM: Ore zones are typically localized in structures, but may occur in permeable lithologies. Upward-flaring ore zones centred on structurally controlled hydrothermal conduits are typical. Large (> 1 m wide and hundreds of metres in strike length) to small veins and stockworks are common with lesser disseminations and replacements. Vein systems can be laterally extensive but ore shoots have relatively restricted vertical extent. High-grade ores are commonly found in dilational zones in faults at flexures, splays and in cymoid loops.

 

TEXTURE/STRUCTURE: Open-space filling, symmetrical and other layering, crustification, comb structure, colloform banding and multiple brecciation.

 

ORE MINERALOGY (Principal and subordinate): Pyrite, electrum, gold, silver, argentite; chalcopyrite, sphalerite, galena, tetrahedrite, silver sulphosalt and/or selenide minerals. Deposits can be strongly zoned along strike and vertically. Deposits are commonly zoned vertically over 250 to 350 m from a base metal poor, Au-Ag-rich top to a relatively Ag-rich base metal zone and an underlying base metal rich zone grading at depth into a sparse base metal, pyritic zone. From surface to depth, metal zones contain: Au-Ag-As-Sb-Hg, Au-Ag-Pb-Zn-Cu, Ag- Pb-Zn. In alkalic hostrocks tellurides, V mica (roscoelite) and fluorite may be abundant, with lesser molybdenite.

 

GANGUE MINERALOGY (Principal and subordinate): Quartz, amethyst, chalcedony, quartz pseudomorphs after calcite, calcite; adularia, sericite, barite, fluorite, Ca- Mg-Mn-Fe carbonate minerals such as rhodochrosite, hematite and chlorite.

 

ALTERATION MINERALOGY: Silicification is extensive in ores as multiple generations of quartz and chalcedony are commonly accompanied by adularia and calcite. Pervasive silicification in vein envelopes is flanked by sericite-illite- kaolinite assemblages. Intermediate argillic alteration [kaolinite-illite- montmorillonite (smectite)] formed adjacent to some veins; advanced argillic alteration (kaolinite-alunite) may form along the tops of mineralized zones. Propylitic alteration dominates at depth and peripherally.

 

WEATHERING: Weathered outcrops are often characterized by resistant quartz ± alunite 'ledges' and extensive flanking bleached, clay-altered zones with supergene alunite, jarosite and other limonite minerals.

 

ORE CONTROLS: In some districts the epithermal mineralization is tied to a specific metallogenetic event, either structural, magmatic, or both. The veins are emplaced within a restricted stratigraphic interval generally within 1 km of the paleosurface. Mineralization near surface takes place in hotspring systems, or the deeper underlying hydrothermal conduits. At greater depth it can be postulated to occur above, or peripheral to, porphyry and possibly skarn mineralization. Normal faults, margins of grabens, coarse clastic caldera moat-fill units, radial and ring dike fracture sets and both hydrothermal and tectonic breccias are all ore fluid channeling structures. Through-going, branching, bifurcating, anastamosing and intersecting fracture systems are commonly mineralized. Ore shoots form where dilational openings and cymoid loops develop, typically where the strike or dip of veins change. Hangingwall fractures in mineralized structures are particularly favourable for high-grade ore.

 

GENETIC MODEL: These deposits form in both subaerial, predominantly felsic, volcanic fields in extensional and strike-slip structural regimes and island arc or continental andesitic stratovolcanoes above active subduction zones. Near- surface hydrothermal systems, ranging from hotspring at surface to deeper, structurally and permeability focused fluid flow zones are the sites of mineralization. The ore fluids are relatively dilute and cool solutions that are mixtures of magmatic and meteoric fluids. Mineral deposition takes place as the solutions undergo cooling and degassing by fluid mixing, boiling and decompression.

 

ASSOCIATED DEPOSIT TYPES: Epithermal Au-Ag: high sulphidation (H04); hotspring Au-Ag (H03); porphyry Cu±Mo±Au (L04) and related polymetallic veins (I05); placer gold (C01, C02).

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Elevated values in rocks of Au, Ag, Zn, Pb, Cu and As, Sb, Ba, F, Mn; locally Te, Se and Hg.

 

GEOPHYSICAL SIGNATURE: VLF has been used to trace structures; radiometric surveys may outline strong potassic alteration of wallrocks. Detailed gravity surveys may delineate boundaries of structural blocks with large density contrasts.

 

OTHER EXPLORATION GUIDES: Silver deposits generally have higher base metal contents than Au and Au-Ag deposits. Drilling feeder zones to hotsprings and siliceous sinters may lead to identification of buried deposits. Prospecting for mineralized siliceous and silica-carbonate float or vein material with diagnostic open-space textures is effective.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: The following data describe the median deposits based on worldwide mines and U.S.A. models:

Au-Ag deposits (41 Comstock-type 'bonanza' deposits) - 0.77 Mt with 7.5 g/t Au, 110 g/t Ag and minor Cu, Zn and Pb. The highest base metal contents in the top decile of deposits all contain <0.1% Cu, Zn and 0.1% Pb
Au-Cu deposits (20 Sado-type deposits) - 0.3 Mt with 1.3% g/t Au, 38 g/t Ag and >0.3% Cu; 10 % of the deposits contain, on average, about 0.75% Cu with one having >3.2% Cu.

 

 

 

 

 

 

 

 

REFERENCES

 

Buchanan, L.J. (1981): Precious Metal Deposits associated with Volcanic Environments in the Southwest; in Relations of Tectonics to Ore Deposits in the Southern Cordillera; Arizona Geological Society Digest, Volume 14, pages 237-262.

Mosier, D.L., Berger, B.R and Singer, D.A. (1986): Descriptive Model of Sado Epithermal Veins; in Mineral Deposits Models, Cox, D.P. and Singer, D.A., Editors, U. S. Geological Survey, Bulletin 1693, page 154.

Mosier, D.L. and Sato, T. (1986): Grade and Tonnage Model of Sado Epithermal Veins; in Mineral Deposits Models, Cox, D.P. and Singer, D.A., Editors, U. S. Geological Survey, Bulletin 1693, pages 155-157.

Mosier, D.L., Singer, D.A. and Berger, B.R (1986): Descriptive Model of Comstock Epithermal Veins; in Mineral Deposits Models, Cox, D.P. and D.A. Singer, D.A., Editors, U. S. Geological Survey, Bulletin 1693, pages 150-153.

Heald, P., Foley, N.K. and Hayba, D.O. (1987): Comparative Anatomy of Volcanic-Hosted Epithermal Deposits: Acid-Sulfate and Adularia Sericite Types; Economic Geology, Volume 82, pages 1-26.

Mosier, D.L., Sato, T., Page, N.J., Singer, D.A. and Berger, B.R. (1986): Descriptive Model of Creede; in Mineral Deposits Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, pages 145-149.

Panteleyev, A. (1991): Gold in the Canadian Cordillera - A Focus on Epithermal and Deeper Deposits; in Ore Deposits, Tectonic and Metallogeny in the Canadian Cordillera, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 163-212.

Sillitoe, R.H. (1993): Epithermal Models: Genetic Types, Geometrical Controls and Shallow Features; in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., Editors, Geological Association of Canada, Special Paper 40, pages 403-417.

White, N.C. and Hedenquist, J.W. (1990): Epithermal Environments and Styles of Mineralization; Variations and their Causes and Guidelines for Exploration; in Epithermal Gold Mineralization of the Circum-Pacific; Geology, Geochemistry, Origin and Exploration, II; Hedenquist, J.W., White, N.C. and Siddeley, G., Editors, Journal of Geochemical Exploration, Volume 36, pages 445-474.

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Sn-Ag VEINS


H07
by A. Panteleyev
British Columbia Geological Survey

 

Panteleyev, A. (1996): Epithermal Au-Ag: Low Sulphidation, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 45-48.

 

IDENTIFICATION

 

SYNONYMS: Polymetallic Sn veins, Bolivian polymetallic veins, polymetallic tin-silver deposits, polymetallic xenothermal.

 

COMMODITIES (BYPRODUCTS): Ag, Sn (Zn, Cu, Au, Pb, Cd, In, Bi, W).

 

EXAMPLES (British Columbia (MINFILE #) - Canada/International): D zone (104P 044, 080,081) and Lang Creek veins (‘Pant’, 104P 082), Cassiar district; Cerro Rico de Potosi, Oruro, Chocaya, (Bolivia), Pirquitas (Argentina), Ashio, Akenobe and Ikuno (Japan).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Sulphide and quartz-sulphide veins carrying cassiterite, a wide variety of other base metals and zones with silver minerals. They are associated with epizonal (subvolcanic) quartz-bearing intrusions, or their immediate hostrocks. In some places the ore is in volcanic rocks within dacitic to quartz latitic flow-dome complexes.

 

TECTONIC SETTING: Continental margin; synorogenic to late orogenic belts with high-level plutonism in intermediate to felsic volcanoplutonic arcs. In British Columbia the only significant Sn-bearing deposits occur with S or A-type granites in eastern tectonic assemblages underlain by continental rocks of North American origin.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: In faults, shears and fractures that cut or are proximal to high-level felsic intrusions and in flow-dome complexes, namely domes and their surrounding tuff rings and explosive breccias.

 

AGE OF MINERALIZATION: Tertiary in the type area of Bolivia; Cretaceous and Tertiary in Japan; Tertiary and older in British Columbia.

 

HOST/ASSOCIATED ROCK TYPES: Hostrocks for veins can be of any type and do not appear to be an important control on the occurrence of the deposits; they include sedimentary, volcanic and intrusive rocks and sometimes, metasedimentary rocks at depth. Intrusive rocks with which the mineralization is associated are quartz bearing and peraluminous, but seem to be restricted to intermediate compositions between 60 and 70% SiO2 (dacite to rhyodacite); more felsic rocks are present, but are less common.

 

DEPOSIT FORM: Veins, commonly with swarms of closely spaced, splaying smaller veins in sheeted zones. Veins vary in width from microveinlets to a few metres, and commonly are less than a metre wide. The ore shoots in veins are commonly 200-300 m along strike and dip but the veins may extend to more than 1000 m in depth and strike length. Vein systems and related stockworks cover areas up to a square kilometre along the tops of conical domes or intrusions 1-2 km wide.

 

TEXTURE/STRUCTURE: Multistage composite banded veins with abundant ore minerals pass at depth into crystalline quartz veins and upwards into vuggy quartz-bearing veins and stockworks.

 

ORE MINERALOGY (Principal and subordinate): Pyrite, cassiterite; pyrrhotite, marcasite; sphalerite, galena, chalcopyrite, stannite, arsenopyrite, tetrahedrite, scheelite, wolframite, andorite, jamesonite, boulangerite, ruby silver (pyrargyrite), stibnite, bismuthinite, native bismuth, molybdenite, argentite, gold and complex sulphosalt minerals. These deposits are characterized by their mineralogical complexity. There is no consistency between deposits in vertical or lateral zoning, but individual deposits are markedly spatially and temporally zoned. In some deposits, notably intrusion or dome-hosted examples, core zones are denoted by the high-temperature minerals cassiterite, wolframite, bismuthinite and arsenopyrite. Surrounding ores have varying amounts of stannite and chalcopyrite with, most significantly, sphalerite, galena and various Pb sulphosalt and Ag minerals. Silver in the upper parts of the vein systems occurs in argentite, ruby silver and native silver and at depth is mainly present in tetrahedrite.

 

GANGUE MINERALOGY (Principal and subordinate): Quartz, sericite, pyrite; tourmaline at depth, kaolinite and chalcedony near surface; rare barite, siderite, calcite, Mn carbonate and fluorite.

 

ALTERATION MINERALOGY: Quartz-sericite-pyrite is characteristic; elsewhere quartz-sericite- chlorite occurs in envelopes on veins. Near-surface argillic and advanced argillic alteration overprinting is present in some deposits.

 

WEATHERING: Prominent limonite cappings are derived from the oxidation of pyrite.

 

ORE CONTROLS: Sets of closely spaced veins, commonly in sheeted zones, fractures and joints within and surrounding plutons are related to the emplacement and cooling of the host intrusions. The open space filling and shear-replacement veins are associated with stockworks, breccia veins and breccia pipes. A few deposits occur in faults, shears, fold axes and cleavage or fracture zones related to regional tectonism. Some early wallrock replacement along narrow fissures is generally followed and dominated by open- space filling in many deposits.

 

GENETIC MODEL: Dacitic magma and the metal-bearing hydrothermal solutions represent the uppermost products of large magmatic/hydrothermal systems. The Sn is probably a remobilized component of sialic rocks derived from recycled continental crust.

 

ASSOCIATED DEPOSIT TYPES: Polymetallic veins Ag-Pb-Zn (I05); epithermal Au-Ag: low sulphidation (H05), mantos (J01, J02), porphyry Sn (L06), placers (C01, C02). This deposit type grades with depth into Sn veins and greissens (I13) associated with mesozonal granitic intrusions into sediments. Cassiterite in colluvium can be recovered by placer mining. Mexican-type rhyolite Sn or “wood tin” deposits represent a separate class of deposit (Reed et al., 1986).

 

COMMENTS: Many Sn-bearing base metal vein systems are known to occur in eastern British Columbia, but there is poor documentation of whether the Sn is present as cassiterite or stannite. The former can be efficiently recovered by simple metallurgy, the latter cannot.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Ag, Cu, Zn, Pb, Sn, W, As, Bi.

 

OTHER EXPLORATION GUIDES: The vein systems may display impressive vertical and horizontal continuity with marked metal zoning. Bolivian polymetallic vein deposits have formed at depths of 0.5 to 2 km below the paleosurface. Deeper veins of mainly massive sulphide minerals contain Sn, W and Bi; the shallower veins with quartz-barite and chalcedony-barite carry Ag and rarely Au. Metal zoning from depth to surface and from centres outward shows: Sn + W, Cu + Zn, Pb + Zn, Pb + Ag and Ag ± Au; commonly there is considerable ‘telescoping’ of zones. Oxidized zones may have secondary Ag minerals, such as Ag chlorides.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Considerable variation in metal contents of ores is evident between deposits. Potentially bulk-mineable bedrock deposits contain in the order of 0.2% Sn with 70-179 g/t Ag (Cerro Rico, Potosi, Bolivia).

 

ECONOMIC LIMITATIONS: These veins tend to be narrow.

 

IMPORTANCE: These veins are an important source of cassiterite for economic placer deposits around the world and the lodes have been mined in South America. They are currently attractive only when they carry appreciable Ag. In some deposits Au content is economically significant and Au-rich zones might have been overlooked during past work. Future Sn production from these veins will probably be as a byproduct commodity, and only if cassiterite is the main Sn mineral.

 

REFERENCES

 

Cunningham, C.G., McNee, J., Pinto Vasquez, J. and Ericksen, G.E. (1991): A Model of Volcanic Dome-hosted Precious Metal Deposits in Bolivia; Economic Geology, Volume 86, pages 415-421.

Ericksen, G.E. and Cunningham, C.G. (1993): Epithermal Precious-metal Deposits Hosted by the Neogene and Quaternary Volcanic Complex in the Central Andes; in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., Editors, Geological Association of Canada, Special Volume 40, pages 419-431.

Grant, J.N., Halls, C., Avila, W., and Avila, G. (1977): Igneous Systems and the Evolution of Hydrothermal Systems in some Sub-volcanic Tin Deposits of Bolivia; in Volcanic Process in Orogenesis, Geological Society of London, Special Paper Publication 7, Pages 117-126.

Ludington, S.D., Orris, G.J., Cox, D.P., Long, K.R. and Asher-Bolinder, S. (1992): Mineral Deposit Models; in Geology and Mineral Resources of the Altiplano and Cordillera Occidental, Bolivia, U.S. Geological Survey, Bulletin 1975, pages 63-89.

Nakamura, T. and Hunahashi, M. (1970): Ore Veins of Neogene Volcanic Affinity in Japan; in Volcanism and Ore Genesis, Tatsumi, T., Editor, University of Tokyo Press, pages 215-230.

Reed, B.L., Duffield, W., Ludington, S.D., Maxwell, C.H. and Richter, D.H. (1986): Descriptive Model of Rhyolite-hosted Sn; in Mineral Deposit Models, U.S. Geological Survey, Bulletin 1693, pages 168-171.

Togashi, Y. (1986): Descriptive Model of Sn-Polymetallic Veins; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin 1693, page 109.

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ALKALIC INTRUSION-ASSOCIATED Au-Ag

H08
by Tom G. Schroeter and Robert Cameron
British Columbia Geological Survey

 

Schroeter, T.G. and Cameron, R. (1996): Alkalic Intrusion-associated Au-Ag, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 49-51.

 

IDENTIFICATION

 

SYNONYMS: Alkalic epithermal, Au-Ag-Te veins.

 

COMMODITIES (BYPRODUCTS): Au, Ag (Zn, Pb).

 

EXAMPLES (British Columbia - Canada/International): Flathead (082GSE070), Howell (082GSE037), Howe (082GSE048); Cripple Creek (Colorado, USA), Zartman, Landusky, Golden Sunlight (Montana, USA), Golden Reward (South Dakota, USA).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: These deposits include quartz veins with pyrite, sphalerite and galena in structural zones and stockworks within alkalic intrusions and/or disseminated pyritic zones in alkalic intrusions, diatremes, coeval volcanics (Cripple Creek) and surrounding sediments. Argillic alteration, +/- silicification, carbonatization, and barite and fluorite veins are common.

 

TECTONIC SETTINGS: Associated with alkalic intrusive rocks in sedimentary cover rocks above continental crust, generally associated with extensional faulting. Tertiary examples in the USA are related to continental rifting; Rio Grande rift for Cripple Creek, Great Falls tectonic zone for the Montana deposits. Flathead area of British Columbia is in a continental setting but the extensional component is not as apparent.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Diatreme-intrusive complexes, high-level alkalic plugs, and dikes that intrude Proterozoic to Mesozoic continental clastic and carbonate rocks. Cripple Creek is within a large maar diatreme complex. Flathead intrusions are coeval with chemically similar volcanic rocks, the Crowsnest volcanics, in southern Alberta.

 

AGE OF MINERALIZATION: Any age; Flathead intrusions are early Cretaceous (98.5 Ma)

 

HOST/ASSOCIATED ROCK TYPES: (Flathead area): Intrusions include alkali feldspar syenite, foid-bearing syenite (nepheline, leucite, nosean, analcite), mela- syenite and related diatreme breccias with 10 % to 100 % intrusive component. Textures include coarse porphyritic sanidine, micro-syenite, tinguaite. Host sedimentary rocks include clastic rocks, shales and argillites to sandstones, and impure fine-grained carbonaceoous limestone and massive calcarenitic limestone. Gold may be present in all rock types.

 

DEPOSIT FORM: Deposits may be in the form of sheeted veins in structural zones within intrusions (e.g, Zortman, Cripple Creek) with dimensions of 50 m to 100 m in width and hundreds of metres in length to, less commonly, large disseminated, diffuse zones within diatremes (e.g., Montana Tunnels, Cripple Creek), volcanic rocks (e.g., Cripple Creek) or stratabound within favourable sedimentary lithologies.

 

TEXTURE/STRUCTURE: Ore minerals in quartz and quartz-adularia veins, vein stockworks, disseminated zones and minor breccias.

 

ORE MINERALOGY (Principal and subordinate): Fine-grained (auriferous, arsenical?) pyrite, galena, sphalerite, gold tellurides; chalcopyrite, magnetite, gold, bismuth and tellurium minerals are suspected at Flathead from elevated geochemical values in samples (to 31 ppm Te, 356 ppm Bi).

 

GAUNGE MINERALOGY (Principal and subordinate): Quartz, calcite; adularia, barite, fluorite.

 

ALTERATION MINERALOGY: Widespread pyrite and carbonate (calcite) alteration of intrusive rocks, silicic and argillic (illite, sericite, jarosite, roscoelite) alteration of wallrocks; also albite and adularia.

 

WEATHERING: Oxidation with limonite, jarosite, hydrozincite.

 

ORE CONTROLS: Mineralization is controlled by structural zones within or proximal to alkalic intrusions; also in permeable (e.g., sandstone) or chemically favourable units (impure carbonates or bedding contacts) in country rocks. Diatreme breccias are favourable permeable hosts for focused flow of volatiles.

 

ASSOCIATED DEPOSIT TYPES: Distal base metal mantos are indicated in the Flathead and South Dakota deposit areas. Possible link with porphyry Mo deposits; polymetallic (I05) veins.

 

COMMENTS: Some authors consider this deposit type to be a subset of the low- sulphidation epithermal suite of precious metal deposits. This deposit model relates to continental rift settings, but related deposit types are present in oceanic arc settings and include Emperor (Fiji), Porgera and Ladolam (Papua New Guinea) deposits. Similar British Columbia settings may include the Quesnel and Stikine Terrane alkalic volcanic belts which host the alkalic porphyry copper-gold deposits (LO3).

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Au, Ag, As, Sb, Pb, Zn, F, Ba, V, Te, Bi

 

GEOPHYSICAL SIGNATURE: High chargeability (I.P.) will outline pyritic zones; magnetic surveys will outline magnetite-bearing zones.

 

ECONOMIC FACTORS:

 

TYPICAL GRADE AND TONNAGE: Highly variable, from very low mineable grades (e.g., 0.53 g/t Au at Zortman) to very high bonanza grades (e.g., 126 g/t Au at the Cresson vug, Cripple Creek). Recovered gold from the Cripple Creek district totals in excess of 600 tonnes. Grades at Howell Creek include 58 m of 1.3 g/t Au in silicified limestone, with grab samples containing up to 184 g/t at Flathead. Tonnages and grades from a number of deposits include: Cresson deposit, Cripple Creek 70 mt 0.99 g/t Au Cripple Creek, historical prod’n (1891-1989) 41 mt 17.14 g/t Au Golden Sunlight (Dec., 1994) 42.8 mt 1.9 g/t Zortman (Dec., 1994) 55.7 mt 0.68 g/t Au Montana Tunnels (Dec., 1994) 26.6 mt 0.61 g/T Au.

 

IMPORTANCE: Although these deposits have not been mined in British Columbia, they remain a viable exploration target.

 

REFERENCES

 

Bonham, H.F. (1988): Models for Volcanic Hosted Epithermal Precious Metal Deposits; in Bulk Mineable Precious Metal Deposits of the Western United States, Schafer, R.W., Cooper, J.J., and Wikre, P.G., Editors, Geological Society of Nevada, Symposium Proceedings, pages 259-271.

Cameron, R.S. (1989): Reverse Circulation Drilling Report for the Howe Claims, Fort Steele Mining Division, British Columbia; B.C. Ministry of Energy, Mines and Petroleum Resources, Assessment Report 18629. Mutschler, G.E. and

Mooney, T.C. (1993): Precious-metal Deposits Related to Alkalic Igneous Rocks: Provisional Classification, Grade-Tonnage Data and Exploration Frontiers; in Mineral Deposit Modelling, R.V. Kirkham, W.D. Sinlcair, R.I. Thorpe and J.M. Duke, Editors, Geological Survey of Canada, Special Paper 40, pages 479- 520.

Richards, J.P. and Kerrick, R. (1993): The Porgera Gold Mine, Papua New Guinea: Magmatic Hydrothermal to Epithermal Evolution of an Alkalic-type Precious Metal Deposit; Economic Geology, Violume 88, pages 1017-1052.

Sillitoe, R.H. (1983): Epithermal Models: Genetic Types, Geometrical Controls and Shallow Features; in Mineral Deposit Modelling, R.V. Kirkham, W.D. Sinlcair, R.I. Thorpe and J.M. Duke, Editors, Geological Survey of Canada, Special Paper 40, pages 403-417.

Skupinski. A. and Legun, A. (1989): Geology of Alkalic Rocks at Twentynine Mile Creek, Flathead River Area, Southeastern British Columbia; in Exploration in British Columbia 1988, B.C. Ministry of Energy, Mines and Petroleum Resources, pages B29- B34.

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HYDROTHERMAL CLAYS


H09
by Z.D. Hora

 

IDENTIFICATION

 

SYNONYMS: Primary kaolin deposits, hypogene kaolin, hydrothermal kaolin, quartz-kaolinite-alunite deposits, Island Arc kaolin model, argillic alteration deposits, epithermal kaolin, hydrothermal alunite.

 

COMMODITIES (BYPRODUCTS): Kaolin, halloysite, pyrophyllite.

 

EXAMPLES (British Columbia - Canada/International):  

Monteith Bay (092L 072, 117, 246, 343), Pemberton Hills (092L 308); Tintic (Utah, USA), Terraced Hills, (Nevada, USA), Matauri Bay, Mahimahi and Maungaparerua (New Zealand),Chugoku, Itaya and Taishu (Japan), Suzhou (China).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION:  Kaolin and halloysite, with or without alunite and pyrophyllite, occurs as veins and massive alteration masses in volcanic and granitic rocks. They formed in geothermal fields and hot springs areas associated with volcanic activity nearby.

 

TECTONIC SETTINGSActive volcanic arcs (oceanic island arcs, continental margin arcs), extensional and transtensional settings, continental margin rifting. Typically the biggest deposits occur in volcanic island arcs, but may develop also in volcanic centres near continental margins.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Near surface hydrothermal alteration zones associated with subaerial volcanic centres and geothermal areas. The volcanic centres can be stratovolcanoes and calderas. Alteration zones typically occur in rocks of higher permeability and to depths of up to 100 metres from the paleosurface.

 

AGE OF MINERALIZATION:  Mostly Tertiary to Quaternary; due to erosion and metamorphism the   older deposits generally have not been preserved. The Suzhou deposit is reported to be related to Jurassic volcanic activity.

 

HOST/ASSOCIATED ROCK TYPES: Rhyolite, trachyte, andesite flows, sometimes glassy, and volcaniclastic rocks and their hypabyssal equivalents. Also, any older basement with feldspathic or sericitic rocks. Associated rocks are hydrothermal breccias and travertine and siliceous sinters.

  

DEPOSIT FORM:  Structurally controlled, cone or wedge-shaped bodies are common; sometimes irregular shapes result from variable host rock permeability controlled by fracture density and porosity.  Many alteration zones spread out as they approach the surface or form large near-surface zones on the flanks of volcanoes. Many of described ore bodies are less than 100 by 200 metres in size. The largest known deposit, Maungaparerua, covers 350 acres and has been explored to 50 m depth.  The Itaya deposit has three zones; the largest one was 300 by 350 metres in plan and 100 metres deep. The Taishu has many orebodies, the largest one being 100 by 200 metres in size.

 

TEXTURE/STRUCTURERelic textures of the original host rock; sometimes aphanitic mixture of clay component with fine-grained silica; stockwork and breccia.

 

ORE MINERALOGY (Principal and subordinate)Halloysite, kaolin, pyrophyllite; dickite, nacrite.

 

GAUNGE MINERALOGY (Principal and subordinate): Quartz, calcite; adularia, barite, fluorite.

 

ALTERATION MINERALOGYClay minerals and pyrophyllite are hydrothermal alteration products of host rocks. This process, which starts with alteration of feldspars and other aluminosilicates, can be pervasive or marginal to fractures. In the latter case, some of the rock will be partially altered with hydrous phyllosilicates surrounding relic feldspar, quartz and mica. The alteration minerals can be zoned outward from core zones of silica through alunite to flanking zones of pyrophyllite-kaolinite-halloysite-smectite-illite Byproduct silica is mobilized and can be precipitated as a silica cap, veins and/or a siliceous matrix to the clay minerals.

 

WEATHERINGCirculating groundwater may further remove leachable elements (K, Na, Ca, Mg, etc.) and improve the quality of clay for a number of end uses. Residual weathering may overprint primary kaolin deposits resulting in superior quality ceramic clay (Maungaparerua, New Zealand).  Silica caps on top of the deposit can form topographic heights.

 

ORE CONTROLSVolcanic centres are a key control.  Diatreme breccias, normal faults, margins of grabens and collapsed calderas can also be loci for some hypogene clay deposits. Alteration zones are often more extensive near the paleosurface.  Hydrothermal clays are often hosted by rocks that are feldspathic, contain felsic glass and/or are permeable.

 

GENETIC MODELS:  Clay deposits develop in feldspathic rocks, with or without volcanic glass, due to the circulation of hydrothermal fluids with low pH values (approximately 3.5 to 5) and temperatures from below 100 to 400°C. Halloysite forms at temperatures under ~100°C, kaolinite and alunite between ~100°C and ~350°C and pyrophyllite between ~300°C and ~400°C. Silica compounds and alunite may precipitate in separate zones, but also as a cementing matrix for kaolinite and pyrophyllite. The deposits occur over a considerable depth, ranging from high temperature geothermal fields at the paleosurface down into cupolas of intrusive bodies at depth.

 

ASSOCIATED DEPOSIT TYPES:  Epithermal Au-Ag - low sulphidation (H05), epithermal Au-Ag-Cu: high sulphidation (H04), hot spring Au-Ag (H03), hot spring Hg (H02), solfatara alteration in vents in modern deposits.

 

COMMENTSIn some cases, near-surface leaching of these alteration zones can produce altered zones containing more than 95% silica. Alunite deposits can be extensive with potential as an aluminum resource, but they are not presently considered to be economic.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATUREHigh aluminum contents and reduced alkali contents, increased silica locally. Presence of aluminum sulphate and/or native sulphur.

 

GEOPHYSICAL SIGNATURE:  Seismic techniques can distinguish dense unaltered rock from less dense clay altered zones. Resistivity methods can identify conductive clay zones from resistive unaltered rocks.

 

OTHER EXPLORATION GUIDES:  Presence of siliceous sinters and association with geothermal fields. Search for alteration zones near volcanic centres and associated fault systems. 

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGEPublished data on individual deposits are very incomplete.  Depending on the original quartz content in the host rock, the clay content may vary from approximately 50 to 80%. Original silica content may be increased by precipitation of mobilized silica released by alteration of feldspars. Only a few hydrothermal deposits are large.

 

ECONOMIC LIMITATIONS:  Production from this deposit type is from open pits. Physical properties and chemical composition of clay affect end use. While some deposits in New Zealand produce high-quality ceramic material, others can be used for white cement only. The high level of processing required to meet industry specifications and transportation cost to the end user are the main limiting factors for kaolin use. While local sources compete for low-value markets, high-quality products may be shipped to users overseas. 

 

END USES:  Hydrothermal kaolins are used in ceramics, for a variety of filler applications (paper, rubber, paints), refractory use and white cement manufacturing. A high content of fine silica makes hydrothermal kaolin hard and unusable for some applications; their main use is in ceramic applications. Deposits with a residual weathering overprint may have kaolin suitable for higher end uses, like industrial fillers and paper coating.

 

IMPORTANCE:  Globally the least important of the three kaolin deposit types, but regionally may be very important (Japan, New Zealand).

 

SELECTED BIBLIOGRAPHY

 

 

Bristow, C.M. (1987): World Kaolins - Genesis, Exploitation and Application; Industrial Minerals, Number 238, pages 45–59.

 

Clapp, C.H. (1914): The Geology of the Alunite and Pyrophyllite Rocks of Kyuquot Sound, Vancouver Island; Summary Report of the Geological Survey for the calendar year 1913,  Ottawa, pages 109–126.

 

Dill, H.G., Bosse, H.R., Henning, K.H., Fricke, A. and Ahrend, H. (1997): Mineralogical and Chemical Variations in Hypogene and Supergene Kaolin Deposits in a Mobile Fold Belt the Central Andes of Northwestern Peru; Mineralium Deposita, Volume 32, pages 149–163.

 

Hall, R.B. (1982): Model for Hydrothermal Alunite Deposits; in Characteristics of Mineral Deposit Occurrences, Erickson, R.L., Editor, United States Geological Survey, Open File Report 82-795, pages 148–154.

 

Harben, P.W. and Kuzwart, M. (1996): Industrial Minerals - A Global Geology; Metal Bulletin, London, 462 pages.

Harvey, C.C. and Murray, H.H. (1997): Industrial Clays in the 21st Century: a Perspective of

 

Exploration, Technology and Utilization; Applied Clay Science, Volume 11, pages 285–310.

 

Malkovsky, M. and Vachtl, J., Editors (1969): Kaolin Deposits of the World, A-Europe, B-Overseas Countries; Proceedings of the Symposium 1, 23rd International Geological Congress, Prague, 1968,

 

460 pages.

 

Malkovsky, M. and Vachtl, J., Editors (1969): Genesis of the Kaolin Deposits; Proceedings of the Symposium 1, 23rd International Geological Congress, Prague, 1988, 135 pages.

 

 

Murray, H.H. (1989): Kaolin Minerals: Their Genesis and Occurrences, Hydrous Phyllosilicates, in Reviews in Mineralogy, Bailey, S.W., Editor, Mineralogical Society of America, Volume 19, pages 67–89.

 

Murray, H.H., Bundy, W.M. and Harvey, C.C., Editors (1993): Kaolin Genesis and Utilization; The Clay Minerals Society, Boulder, Colorado, 341 pages.

 

Panteleyev, A. and Koyanagi, V.M. (1994): Advanced Argillic Alteration in Bonanza Volcanic Rocks, Northern Vancouver Island - Lithologic and Permeability Controls; British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1994-1, pages 101-110.

 

*  Note:  All BC deposit profile #s with an asterisk have no completed deposit profile.  USGS deposit model #s with an asterisk had no published model in the late 1990s.

Examples of Epithermal Deposits

BC Profile # Global Examples B.C. Examples
 H01 Gardiner (Montana), Salida (Colorado), Lazio (Italy) Clinton, Slocan, Deep River
 H02 Sulphur Bank (California), Steamboat Springs (Nevada) Ucluelet
 H03 McLaughlin (California), Round Mountain (Nevada) Cinola, Clisbako, Wolf?, Trout?
 H04 El Indio (Chile), Nansatsu (Japan) Westpine, Taylor-Windfall, Mt. McIntosh
 H05 Comstock (Nevada), Sado (Japan) Lawyers, Blackdome, Silbak Premier
   H06* Talamantes (Mexico), Gloryana (New Mexico) - -
 H07 Black Range (New Mexico), Potosi (Bolivia), Ashio (Japan) D Zone and Lang Creek (Cassiar)
 H08 Emperor (Fiji), Cripple Creek (Colorado), Zortman (Montana) Flathead, Howell, Howe
 H09 Cornwall (England) Monteith Bay, Pemberton Hills