Dark Hill

Dark Hill Molybdenum-Tungsten  +/- Ag-Sn

IDENTIFICATION SYNONYM: Stockwork W-Mo COMMODITIES (BYPRODUCTS): W (Mo, Sn, Ag). EXAMPLES (British Columbia - Canada/International): Boya; Mount Pleasant (New Brunswick, Canada), Logtung (Yukon, Canada), Xingluokeng, Lianhuashan and Yanchuling (China). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Stockwork of W-bearing quartz veinlets and fractures in felsic intrusive rocks and associated country rocks. Deposits are low grade but large and amenable to bulk mining methods. TECTONIC SETTING: Zones of weak to moderate extension in cratons, particularly post- collisional zones in areas of tectonically thickened crust. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: High-level to subvolcanic felsic intrusive centres; multiple stages of intrusion are common. AGE OF MINERALIZATION: Paleozoic to Tertiary, but Mesozoic and Tertiary examples are more common. HOST/ASSOCIATED ROCK TYPES: Highly variable; mineralized rocks may be predominantly genetically related intrusive rocks, but may also be related or unrelated sedimentary, volcanic, igneous and metamorphic rocks. Genetically related felsic intrusive rocks are commonly F-rich (fluorite and/or topaz bearing) and porphyritic; unidirectional solidification features, particularly comb quartz layers, may also be present. Tuffs or other extrusive volcanic rocks may be associated with deposits related to subvolcanic intrusions. DEPOSIT FORM: Deposits vary in shape from inverted cup-shaped, to roughly cylindrical, to highly irregular. They are typically large, generally hundreds of metres across and ranging from tens to hundreds of metres in vertical extent. TEXTURE/STRUCTURE: Ore minerals is structurally controlled; mainly stockworks of crosscutting fractures and quartz veinlets, also veins, vein sets, breccias, disseminations and replacements. ORE MINERALOGY (Principal and subordinate): Main ore mineral is generally either scheelite or wolframite, although in some deposits both are present. Subordinate ore minerals include molybdenite, bismuth, bismuthinite and cassiterite. GANGUE MINERALOGY: Pyrite, pyrrhotite, magnetite, arsenopyrite, löllingite, quartz, K- feldspar, biotite, muscovite, fluorite, topaz. ALTERATION MINERALOGY: Hydrothermal alteration is pervasive to fracture controlled and, at deposit scale, is concentrically zoned. It is commonly characterized by the presence of greisen alteration minerals, including topaz, fluorite and Li- and F-rich micas. At Mount Pleasant, for example, pervasive greisen alteration consisting of quartz + topaz ± sericite ± chlorite associated with high-grade W zones and grades laterally into fracture-controlled quartz- biotite-chlorite-topaz alteration associated with lower grade W zones. Propylitic alteration, mainly chlorite and sericite, extends as far as 1500 m beyond the mineralized zones. Potassic alteration, dominated by K-feldspar, occurs locally within the central areas of pervasive greisen alteration. Other deposits such as Xingluokeng (China) are characterized more by central zones of silicic and potassic alteration (K-feldspar and biotite); zones of weak greisen alteration consisting of muscovite and fluorite may be present. Sericitic alteration forms a broad aureole around the central potassic zone; irregular zones of argillic alteration may be superimposed on both the potassic and sericitic zones. In detail, alteration patterns may be complex; at Logtung, for example, different stages of mineralized veins and fractures are characterized by different assemblages of ore and alteration minerals. WEATHERING: Oxidation of pyrite produces limonitic gossans; oxidation of molybdenite, if present, may produce yellow ferrimolybdenite. ORE CONTROLS: Quartz veinlet and fracture stockwork zones surround or are draped over and are superimposed to varying degrees on small stocks (<1 km2); multiple stages of mineralization commonly present; felsic intrusions associated with the deposits are typically F-rich. GENETIC MODEL: Magmatic-hydrothermal. Large volumes of magmatic, highly saline aqueous fluids under pressure strip W, Mo and other ore metals from temporally and genetically related magma. Multiple stages of brecciation related to explosive fluid pressure release from the upper parts of small intrusions result in deposition of ore and gangue minerals in crosscutting fractures, veinlets and breccias in the outer carapace of the intrusions and associated country rocks. Incursion of meteoric water during waning stages of the magmatic-hydrothermal system may result in late alteration of the hostrocks, but does not play a significant role in the ore forming process. ASSOCIATED DEPOSIT TYPES: Porphyry W deposits may be part of a spectrum of deposits that include Climax-type Mo deposits (L08) as one end-member and porphyry Sn deposits as the other (L06). Vein/replacement W, Sn, Ag deposits may be associated (I05, H07), e.g. Logjam Ag-Pb-Zn veins peripheral to the Logtung W-Mo deposit. Skarn (contact metamorphic) zones associated with genetically related felsic intrusions may be mineralized, but are not typical skarn W (i.e. contact metasomatic) deposits. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: W, Mo and Sn are anomalous in hostrocks close to mineralized zones; anomalously high contents of F, Zn, Pb and Cu occur in wallrocks up to several kilometres from mineralized zones. W, Sn, Mo, F, Cu, Pb and Zn may be anomalously high in stream sediments and W, Sn and F (topaz) may be present in heavy mineral concentrates. GEOPHYSICAL SIGNATURE: Genetically related intrusions may be magnetic lows (ilmenite rather than magnetite dominant); contact aureole may be magnetic high if pyrrhotite or magnetite are present in associated skarn or hornfels zones. Radiometric surveys may be used to outline anomalous U, Th or K in genetically related intrusive rocks or in associated altered and mineralized zones. OTHER EXPLORATION GUIDES: The presence of scheelite can be detected with an ultraviolet lamp. ECONOMIC FACTORS GRADE AND TONNAGE: Tens to more than 100 Mt at grades of 0.2 to 0.3 % W (Lianhushan is exceptional at 0.8 % W). Boya (British Columbia): limited size due to thrust fault truncation, no published resource data. Mount Pleasant (New Brunswick): Fire Tower zone: 22.5 Mt @ 0.21 % W, 0.10 % Mo, 0.08 % Bi, (includes 9.4 Mt @ 0.31 % W, and 0.12 % Mo), North zone: 11 Mt @ 0.2 % W, 0.1 % Mo. Logtung (Yukon): 162 Mt @ 0.10 % W, 0.03 % Mo. Xingluokeng (China): 78 Mt @ 0.18 % W. Lianhuashan (China): ~40 Mt @ 0.8 % W. ECONOMIC LIMITATIONS: Low grades require high production volumes which may not be justified by current demand for tungsten. IMPORTANCE: Not currently an important source of world W production; some W may be recovered from deposits in China (e.g. Lianhuashan), but none is recovered at present (1994) from deposits outside China. Mount Pleasant Tungsten in New Brunswick produced slightly more than 2000 t of concentrate grading 70 % WO3 from 1 Mt of ore mined from 1983 to 1985. REFERENCES Kirkham, R.V. and Sinclair, W.D. (1984): Porphyry Copper, Molybdenum, Tungsten; in Canadian Mineral Deposit Types: A Geological Synopsis; Geological Survey of Canada, Economic Geology Report 36, pages 51-52. Kirkham, R.V. and Sinclair, W.D. (1988): Comb Quartz Layers in Felsic Intrusions and their Relationship to Porphyry Deposits; in Recent Advances in the Geology of Granite-related Mineral Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 39, pages 50-71. Kooiman, G.J.A., McLeod, M.J. and Sinclair, W.D. (1986): Porphyry Tungsten-Molybdenum Orebodies, Polymetallic Veins and Replacement Bodies, and Tin-bearing Greisen Zones in the Fire Tower Zone, Mount Pleasent, New Brunswick; Economic Geology, Volume 81, pages 1356-1373. Liu, W. (1980): Geological Features of Mineralization of the Xingluokeng Tungsten (Molybdenum) Deposit, Fujian Province; in Tungsten Geology, China, Hepworth, J.V. and Lu, H.Z., Editors, ESCAP/RMRDC, Bandung, Indonesia, pages 338-348. Noble, S.R., Spooner, E.T.C. and Harris, F.R. (1986): Logtung: A Porphyry W-Mo Deposit in the Southern Yukon; in Mineral Deposits of Northern Cordillera, Morin, J.A., Editor, Canadian Institute of Mining and Metallurgy, Special Volume 37, pages 274- 287. Sinclair, W.D. (1986): Molybdenum, Tungsten and Tin Deposits and associated Granitoid Intrusions in the Northern Canadian Cordillera and adjacent parts of Alaska; in Mineral Deposits of Northern Cordillera, Morin, J.A., Editor, Canadian Institute of Mining and Metallurgy, Special Volume 37, pages 216-233. Yan, M-Z., Wu, Y-L. and Li, C.-Y. (1980): Metallogenetic Systems of Tungsten in Southeast China and their Mineralization Characteristics; in Granitic Magmatism and Related Mineralization, Ishihara, S. and Takenouchi, S., Editors, The Society of Mining Geologists of Japan, Mining Geology Special Issue, No. 8, pages 215-221.