China Valley Minerals: Dark Hill

Dark Hill from Charcoal Creek

Dark Hill mineralization is related to a system of Cretaceous subvolcanic epizonal felsic intrusions and associated Rhyolites hosted in Paleozoic marine sediments. Several types of mineralization exist including porphyry W+Mo +/- Sn Ag, crosscutting sulphide veins, breccias, skarn, disseminations and replacements. The Paleozoic country rocks have undergone regional transtensional rifting and rapid infilling. Hydraulic fracturing is pervasive and facilitated the rapid exsolution of ore shoots. Sulphide veins cutting porphyry mineralization illustrates several phases of mineralization related to multiple intrusions. The deposit shows many similarities to the Logtung porphyry Mo deposit in the western Cordillera and may be classed as a porphyry W(scheelite)-Mo deposit. The principal reasons are that mineralization is spatially associated with porphyritic intrusive phases and that much of the mineralization occurs as typical porphyry-style crackle breccias.

PORPHYRY W (L07)

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 anassemblages of ore and alteration minerals.