Deposit Models - Mesothermal Gold VeinsMINERAL DEPOSIT PROFILES
MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITSIN BRITISH COLUMBIA OCEANIC TERRANES
THE SNIP AND JOHNNY MOUNTAIN GOLD MINES: EARLY JURASSIC INTRUSIVE-RELATED VEIN DEPOSITS, ISKUT RIVER AREA, NORTHWESTERN BRITISH COLUMBIA
INTRUSION-RELATED AU-(AG-CU) PYRRHOTITE VEINS
THE SNIP AND JOHNNY MOUNTAIN GOLD MINES: EARLY JURASSIC INTRUSIVE-RELATED VEIN DEPOSITS, ISKUT RIVER AREA, NORTHWESTERN BRITISH COLUMBIA
INTRUSION-RELATED AU-(AG-CU) PYRRHOTITE VEINS
C. H. Ash, Geological Survey, B.C. Ministry of Energy, Mines and Petroleum Resources; P.H. Reynolds, Department of Geological Sciences Dalhousie University and R.W.J. Macdonald, Mineral Deposit Research Unit, Department of Geological Sciences, The University of British Columbia
Mesothermal gold quartz vein deposits in British Columbia (eg. Bralorne-Pioneer and Cassiar) and gold placer deposits derived from such veins (eg. Atlin, Cariboo, Dease Lake and Manson Creek) are, or were hosted within or marginal to collisional suture zones where large volumes of CO2-rich fluids have been channeled. These zones represent major crustal breaks between diverse assemblages of island arcs, subduction complexes and continental margin clastic wedges. They are delineated by the presence of obducted remnants of ancient oceanic lithosphere, i.e. dismembered ophiolitic rocks.
Deposits are intimately associated with carbonate altered ultramafic rocks "listwanite" derived from oceanic lower crustal plutonic or upper mantle metamorphic protoliths. The presence of such ultramafic rocks at surface, in essence characterize the trans-crustal nature of these major fault zones. Listwanite is therefore significant in that it delineates such suture zones and, more importantly marks areas where the sutures have channeled potential mineralizing fluids.
Gold mineralization is characterized by silicification, pyritization and potassic metasomatism localized along fracture zones within broader carbonate alteration halos. Economic concentrations, due to the likelihood of vein continuity and definable reserves are most likely hosted by the more competent lithologies of the obducted oceanic lithosphere, which form relatively large tectonic blocks. The differentiated mafic plutonic oceanic crustal segment of the East Lisa Complex ("Bralorne Intrusion" or "Bralorne Diorite") hosting the Bralorne gold veins and the upper crustal volcanic rocks of the Sylvester allochthon hosting the Erickson gold veins are British Columbia examples. The Grass Valley district in the Motherlode Belt was the richest and most famous gold mining district in California, with practically all the gold recovered from lodes. As at Bralorne, the veins are hosted in a mafic plutonic-volcanic section of obducted crust, the Smartville Complex.
These veins appear to form during periods of metamorphism and partial melting due to tectonic crustal thickening in response to arc-continent collision. They are typically associated with late syn-collisional intermediate to felsic magmatism. Mineralizing hydrothermal fluids are interpreted to be derived, at least in part, from tectonically thickened, hydrated oceanic lithosphere that undergoes metamorphic dehydration and partial melting during and after faulting.
Ar39/Ar40 ages of hydrothermal vein mica from the Cache Creek and Bridge River Terrane define temporally restricted mineralizing events which closely follow a collisional episode. In contrast, published K/Ar data for deposits associated with the Slide Mountain Terrane suggest that mineralization was temporally much less restrictive and formed during a period of uplift and extension in Early Cretaceous.
The available age data suggest that either:
* There are two distinct tectonic regimes of mesothermal gold-quartz vein formation in the Cordillera, one involving a collisional event and the other produced during extension and uplift, or that
* All these vein deposits are late-syncollisional and the K-Ar systematics of mesothermal vein deposits occurring in association with oceanic lithosphere above the American continental margin have been reset by later thermal events.
Mesothermal gold quartz vein deposits are found along suture zones where affected by intense and pervasive carbonate alteration that is closely associated with late syn-collisional, structurally controlled intermediate to felsic magmatism They are potentially economic where hosted by relatively large, competent tectonic blocks of obducted oceanic crust.
P - THE SNIP AND JOHNNY MOUNTAIN GOLD MINES: EARLY JURASSIC INTRUSIVE-RELATED VEIN DEPOSITS, ISKUT RIVER AREA, NORTHWESTERN BRITISH COLUMBIA David A. Rhys, Consulting Geologist
The Snip and Johnny Mountain gold mines occur five kilometres apart on Johnny Mountain in northwestern British Columbia. The area is underlain by in a folded sequence of Upper Triassic turbiditic and volcaniclastic rocks, which host the Snip mine. The Triassic rocks are unconformably overlain by flat lying Early Jurassic volcanic rocks at the Johnny Mountain mine.
Ore at the Snip mine occurs in two southwest-dipping shear veins, the Twin zone and its splay, the 150 vein, which together contain >30 tonnes Au. The deposit comprises interlayered (i) laminated calcite and chlorite-biotite-pyrite replacement shear veins and (ii) dilatant quartz and pyrite-pyrrhotite veins. Veins were emplaced progressively during normally-directed simple shear that accompanied a period of semi-brittle deformation.
The Johnny Mountain mine (Stonehouse deposit, 3 tonnes Au production) located south of Snip, consists of a set of steep north-dipping dilatant quartz-pyrite veins with K-feldspar alteration envelopes. The veins are superimposed on flat lying Early Jurassic volcaniclastic rocks that are intruded by a series of Early Jurassic feldspar porphyry dykes. Structural relations suggest that the Stonehouse veins represent a higher level, more brittle response to the same deformational event that formed the stratigraphically deeper Snip orebodies.
The Early Jurassic Red Bluff K-feldspar megacrystic quartz diorite stock intrudes Triassic rocks 300-800 metres northeast of the Twin zone. The intrusion is affected by (i) early intense quartz-magnetite-sericite-K-feldspar-biotite (potassic) alteration associated with abundant quartz-magnetite-hematite veins and Au-Cu-Mo mineralization, overprinted by (ii) sericite-pyrite-quartz (phyllic) alteration characterised by pyrite veining. Geologic relations, including similarities in alteration and structural style, geochronology, and camp-scale mineralogic and alteration zoning, indicate that intrusion, deformation, initiation of the porphyry hydrothermal system, and formation of the structurally hosted Au and base metal deposits are closely related spatially, temporally and probably genetically.
Q - INTRUSION-RELATED AU-(AG-CU) PYRRHOTITE VEINS Dani Alldrick, British Columbia Geological Survey
Intrusion-related gold-bearing pyrrhotite veins occur as a series of parallel, tabular to cymoid veins of massive iron sulphide and/or bull quartz. These moderate tonnage, high-grade veins are emplaced in en echelon fracture sets around the periphery of subvolcanic plutons. Examples of this newly-recognized deposit-type include some of the historic gold camps of British Columbia. These deposits are attractive exploration targets because of their high profit potential (high grades), ease of mining (strong, regular, structural control), relative ease of exploration (predictable restricted geologic setting; characteristic geophysical response) and high exploration potential (deposits occur in clusters or sets of veins and also have close genetic associations with other important mineral deposit types).
Veins may be composed of (i) massive fine-grained pyrrhotite and/or pyrite, or (ii) massive bull quartz with minor calcite and minor to accessory disseminations, knots and crystal aggregates of sulphides. These two dominant vein types may occur independently or together. The two mineralization styles may grade into each other along a vein, may form parallel to each other in a compound vein, or they may occur in adjacent but separate veins within an en echelon set.
The subvolcanic setting for these deposits is transitional between the setting for porphyry copper systems and the setting for epithermal systems. Mineralization is synvolcanic and syn-intrusive and formed along the thermally "brittle-ductile transition envelope" that surrounds subvolcanic intrusions. Late magma movement generated localized shearing which opened en echelon vein sets. Circulating hydrothermal fluid precipitated gold-rich iron sulphides and gangue.
All examples of this deposit type are emplaced in volcanic arc environments in oceanic or continental margin settings. These deposits have close associations with other ore deposits that are typical of arc environments. Consequently intrusion-related Au-(Ag-Cu) pyrrhotite veins should provide new exploration targets within established arc-related porphyry and epithermal camps. Conversely, discovery of these high-grade gold veins in frontier areas should spur exploration for additional deposits of this type, and for all the associated mineral deposit types of the volcanic arc environment.
Last Updated June 13, 2003
Mesothermal gold quartz vein deposits in British Columbia (eg. Bralorne-Pioneer and Cassiar) and gold placer deposits derived from such veins (eg. Atlin, Cariboo, Dease Lake and Manson Creek) are, or were hosted within or marginal to collisional suture zones where large volumes of CO2-rich fluids have been channeled. These zones represent major crustal breaks between diverse assemblages of island arcs, subduction complexes and continental margin clastic wedges. They are delineated by the presence of obducted remnants of ancient oceanic lithosphere, i.e. dismembered ophiolitic rocks.
Deposits are intimately associated with carbonate altered ultramafic rocks "listwanite" derived from oceanic lower crustal plutonic or upper mantle metamorphic protoliths. The presence of such ultramafic rocks at surface, in essence characterize the trans-crustal nature of these major fault zones. Listwanite is therefore significant in that it delineates such suture zones and, more importantly marks areas where the sutures have channeled potential mineralizing fluids.
Gold mineralization is characterized by silicification, pyritization and potassic metasomatism localized along fracture zones within broader carbonate alteration halos. Economic concentrations, due to the likelihood of vein continuity and definable reserves are most likely hosted by the more competent lithologies of the obducted oceanic lithosphere, which form relatively large tectonic blocks. The differentiated mafic plutonic oceanic crustal segment of the East Lisa Complex ("Bralorne Intrusion" or "Bralorne Diorite") hosting the Bralorne gold veins and the upper crustal volcanic rocks of the Sylvester allochthon hosting the Erickson gold veins are British Columbia examples. The Grass Valley district in the Motherlode Belt was the richest and most famous gold mining district in California, with practically all the gold recovered from lodes. As at Bralorne, the veins are hosted in a mafic plutonic-volcanic section of obducted crust, the Smartville Complex.
These veins appear to form during periods of metamorphism and partial melting due to tectonic crustal thickening in response to arc-continent collision. They are typically associated with late syn-collisional intermediate to felsic magmatism. Mineralizing hydrothermal fluids are interpreted to be derived, at least in part, from tectonically thickened, hydrated oceanic lithosphere that undergoes metamorphic dehydration and partial melting during and after faulting.
Ar39/Ar40 ages of hydrothermal vein mica from the Cache Creek and Bridge River Terrane define temporally restricted mineralizing events which closely follow a collisional episode. In contrast, published K/Ar data for deposits associated with the Slide Mountain Terrane suggest that mineralization was temporally much less restrictive and formed during a period of uplift and extension in Early Cretaceous.
The available age data suggest that either:
* There are two distinct tectonic regimes of mesothermal gold-quartz vein formation in the Cordillera, one involving a collisional event and the other produced during extension and uplift, or that
* All these vein deposits are late-syncollisional and the K-Ar systematics of mesothermal vein deposits occurring in association with oceanic lithosphere above the American continental margin have been reset by later thermal events.
Mesothermal gold quartz vein deposits are found along suture zones where affected by intense and pervasive carbonate alteration that is closely associated with late syn-collisional, structurally controlled intermediate to felsic magmatism They are potentially economic where hosted by relatively large, competent tectonic blocks of obducted oceanic crust.
P - THE SNIP AND JOHNNY MOUNTAIN GOLD MINES: EARLY JURASSIC INTRUSIVE-RELATED VEIN DEPOSITS, ISKUT RIVER AREA, NORTHWESTERN BRITISH COLUMBIA David A. Rhys, Consulting Geologist
The Snip and Johnny Mountain gold mines occur five kilometres apart on Johnny Mountain in northwestern British Columbia. The area is underlain by in a folded sequence of Upper Triassic turbiditic and volcaniclastic rocks, which host the Snip mine. The Triassic rocks are unconformably overlain by flat lying Early Jurassic volcanic rocks at the Johnny Mountain mine.
Ore at the Snip mine occurs in two southwest-dipping shear veins, the Twin zone and its splay, the 150 vein, which together contain >30 tonnes Au. The deposit comprises interlayered (i) laminated calcite and chlorite-biotite-pyrite replacement shear veins and (ii) dilatant quartz and pyrite-pyrrhotite veins. Veins were emplaced progressively during normally-directed simple shear that accompanied a period of semi-brittle deformation.
The Johnny Mountain mine (Stonehouse deposit, 3 tonnes Au production) located south of Snip, consists of a set of steep north-dipping dilatant quartz-pyrite veins with K-feldspar alteration envelopes. The veins are superimposed on flat lying Early Jurassic volcaniclastic rocks that are intruded by a series of Early Jurassic feldspar porphyry dykes. Structural relations suggest that the Stonehouse veins represent a higher level, more brittle response to the same deformational event that formed the stratigraphically deeper Snip orebodies.
The Early Jurassic Red Bluff K-feldspar megacrystic quartz diorite stock intrudes Triassic rocks 300-800 metres northeast of the Twin zone. The intrusion is affected by (i) early intense quartz-magnetite-sericite-K-feldspar-biotite (potassic) alteration associated with abundant quartz-magnetite-hematite veins and Au-Cu-Mo mineralization, overprinted by (ii) sericite-pyrite-quartz (phyllic) alteration characterised by pyrite veining. Geologic relations, including similarities in alteration and structural style, geochronology, and camp-scale mineralogic and alteration zoning, indicate that intrusion, deformation, initiation of the porphyry hydrothermal system, and formation of the structurally hosted Au and base metal deposits are closely related spatially, temporally and probably genetically.
Q - INTRUSION-RELATED AU-(AG-CU) PYRRHOTITE VEINS Dani Alldrick, British Columbia Geological Survey
Intrusion-related gold-bearing pyrrhotite veins occur as a series of parallel, tabular to cymoid veins of massive iron sulphide and/or bull quartz. These moderate tonnage, high-grade veins are emplaced in en echelon fracture sets around the periphery of subvolcanic plutons. Examples of this newly-recognized deposit-type include some of the historic gold camps of British Columbia. These deposits are attractive exploration targets because of their high profit potential (high grades), ease of mining (strong, regular, structural control), relative ease of exploration (predictable restricted geologic setting; characteristic geophysical response) and high exploration potential (deposits occur in clusters or sets of veins and also have close genetic associations with other important mineral deposit types).
Veins may be composed of (i) massive fine-grained pyrrhotite and/or pyrite, or (ii) massive bull quartz with minor calcite and minor to accessory disseminations, knots and crystal aggregates of sulphides. These two dominant vein types may occur independently or together. The two mineralization styles may grade into each other along a vein, may form parallel to each other in a compound vein, or they may occur in adjacent but separate veins within an en echelon set.
The subvolcanic setting for these deposits is transitional between the setting for porphyry copper systems and the setting for epithermal systems. Mineralization is synvolcanic and syn-intrusive and formed along the thermally "brittle-ductile transition envelope" that surrounds subvolcanic intrusions. Late magma movement generated localized shearing which opened en echelon vein sets. Circulating hydrothermal fluid precipitated gold-rich iron sulphides and gangue.
All examples of this deposit type are emplaced in volcanic arc environments in oceanic or continental margin settings. These deposits have close associations with other ore deposits that are typical of arc environments. Consequently intrusion-related Au-(Ag-Cu) pyrrhotite veins should provide new exploration targets within established arc-related porphyry and epithermal camps. Conversely, discovery of these high-grade gold veins in frontier areas should spur exploration for additional deposits of this type, and for all the associated mineral deposit types of the volcanic arc environment.
Last Updated June 13, 2003
Gold, the noblest of metals, has been used by man for more than 5000 years. Its extreme softness or malleability, and resistance to tarnish (oxidation), led to its earliest uses in art and currency. Gold is the metal of choice for jewellery, and is often used in dentistry. Gold has also been used successfully in many modern technological applications. It is used as the electrical contacts of computer chips. Minute quantities of gold (less than 3 micrograms) are vaporized to mirror lens surfaces. The intrinsic value of gold offers an attractive alternative to stocks and bonds for many investors. It remains the principle medium for setting currency values and settling international debts among the nations of the world.Minor concentrations of gold occurs in most natural substances. In seawater, for example, there is approximately 0.012 parts per billion (ppb) of gold, and in fresh water it is slightly higher at 0.02 ppb. Its average concentration in the Earth's crust or lithosphere is approximately 5 ppb, and in certain sedimentary rocks it may achieve concentrations of up to 2100 ppb or 2.1 parts per million (ppm). At these concentrations 20 or 30 tons of rock must be processed to extract a single ounce of precious gold. As a result, gold can only be mined profitably where it is highly concentrated by natural chemical and physical processes.Gold occurs in many different geologic settings and its classification into deposit types is complicated. However, two basic types of occurrences or deposits are recognized, primary and secondary. Both rely on similar chemical and physical processes to produce economic concentrations of gold ore. Primary deposits form where gold precipitates during chemical reactions between hydrothermal (hot fluids) mineralizing solutions (metal-bearing)and rocks in the Earth's crust. Secondary deposits form later during the chemical and mechanical processes of weathering and erosion, and the physical reconcentration of gold-bearing sediment into placer deposits.Hydrothermal deposits can be classified as either epigenetic (deposits that form after the formation of the surrounding rocks and other events of mineralization) or syngenitic (deposits that form the same time as surrounding rocks). In epigenetic hydrothermal deposits gold may occur as the principle metal or as a secondary mineral associated with other metals, such as iron, copper, lead and zinc. In these epigenetic hydrothermal deposits. One variety of epigenetic deposit (epithermal gold deposits) form at temperatures below 350°C by the convective circulation of fluids to depths of approximately 2 kilometres, usually near hot igneous bodies or plutons in volcanically active regions. In this type of hydrothermal occurrence gold is generally at relatively low concentrations. Hot springs are modern examples of this type of mineralization process. Mesothermal gold deposits, which form at temperatures above 350°C, occur along large breaks or faults in continental crust. The origin of these is not certain, but they form at depths of 3 to 5 kilometres below the Earth's crust, and appear to be associated with the upward migration of fluids from the Earth's mantle.Gold is often extracted as a by-product during smelting from volcanogenic massive sulphide (VMS) deposits (syngenetic hyrdothermal deposits) which are generated by the accumulation of metal-rich sediments near active volcanic centres on the seafloor. Gold is also found in porphyry copper deposits, high volume (up to 1000 million tons), low grade (0.7% Cu) deposits, formed by the circulation of fluids through the Earth's crust during the volcanic activity related to mountain building above active subduction zones. Secondary gold occurrences or placer deposits are formed by the deposition and reconcentration of gold-bearing sediments from primary gold occurrences. Placer deposits are generally classified according to their depositional environment. Marine placers occur offshore near coastlines; fluvial placers occur in river and stream valleys in the drainage basins which contain primary gold occurrences upstream. Some studies suggest that gold is not only mechanically transported in placers, but that it is also chemically transported. The unusual size and purity of nuggets in some placer deposits supports this theory for gold transportation.Why is it important to continue the search for gold and other metals? Aside from the obvious financial benefits associated with the discovery and development of mineral deposits to mining companies, there are many benefits to communities fortunate enough to be located near producing mines. Exploration and mine development are activities that create jobs. They require highly trained professionals, and skilled technical personnel that may be found in local communities. In addition to the manpower and labour requirements, mineral property and mine development activities often require additional materials and specialized technical services. These are often provided by geologic and mining engineering companies, who locate offices in local communities to participate in exploration and mine development contracts. The economic "spin-offs" to communities from these activities are often significant. Resource-based activities, including mining and exploration often serve as the base for local and regional economies.Where is the gold in Newfoundland? Despite an extensive exploration and mining history, gold exploration is a relatively new activity in Newfoundland and Labrador. While the gold-bearing base metal VMS deposits, such as at the former Buchans and Rambler mines, are well known, exploration over the last two decades has resulted in the recognition of numerous epigenetic gold deposits and prospects in the province. Important recent discoveries include the Hope Brook Mine, near Burgeo, and the Pine Cove and Nugget Pond deposits, near Baie Verte. There is also excellent potential for the discovery of marine placers in regions, such as the Baie Verte Peninsula, with an abundance of primary base metal and gold deposits. The potential for fluvial placers has not been investigated, but these may be discovered in some of the larger river basins.To summarize, recent exploration in Newfoundland and Labrador has resulted in the discovery of several new, significant gold prospects. While many of these are sub-economic, a few have been successfully developed as mines. The current economic recovery in Canada, and increases in the market price of gold have already resulted in increases in the level of prospecting and exploration in Newfoundland and Labrador, activities which may lead to the development of these gold deposits. Gold should be seriously considered as a mineral commodity of great importance to the development and economic diversification of Newfoundland and Labrador. This is the ancient alchemic symbol for gold. Innumerable experiments which were focused on transforming base metals and other materials to gold made significant contributions to the science of chemistry.Further Reading
Boyle, R. W.
1979: The geochemistry of gold and its deposits. Geological Survey of Canada, Bulletin 280, 583 pages.
Brimhall, G.
1991: The genesis of ores. Scientific American, May, p. 84-91.
Rona, P. A.
1992: Deep-sea geysers of the Atlantic. National Geographic, vol. 184, no. 3, p. 105-109.
Roberts, R. G., and Sheahan, P. A.
1988: Ore deposit models. Geoscience Canada Reprint, Series 3, 194 pages.
Swinden, H. S.
1991: Regional geology and metallogeny of Central Newfoundland. In Swinden, H. S., Evans, D. W. T., and Kean, B. F. (editors) Metallogenic framework of base and precious metal deposits, Central and Western Newfoundland (Field Trip 1), Geological Survey of Canada Open File 2156, p. 7-19.
Tuach, J.
1990: List of gold occurrences and deposits in Newfoundland. Newfoundland Department of Mines and energy, Open File 1928, 72 pages.
Boyle, R. W.
1979: The geochemistry of gold and its deposits. Geological Survey of Canada, Bulletin 280, 583 pages.
Brimhall, G.
1991: The genesis of ores. Scientific American, May, p. 84-91.
Rona, P. A.
1992: Deep-sea geysers of the Atlantic. National Geographic, vol. 184, no. 3, p. 105-109.
Roberts, R. G., and Sheahan, P. A.
1988: Ore deposit models. Geoscience Canada Reprint, Series 3, 194 pages.
Swinden, H. S.
1991: Regional geology and metallogeny of Central Newfoundland. In Swinden, H. S., Evans, D. W. T., and Kean, B. F. (editors) Metallogenic framework of base and precious metal deposits, Central and Western Newfoundland (Field Trip 1), Geological Survey of Canada Open File 2156, p. 7-19.
Tuach, J.
1990: List of gold occurrences and deposits in Newfoundland. Newfoundland Department of Mines and energy, Open File 1928, 72 pages.
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