The Highland Valley porphyry district, in southern British Columbia, has jive major porphyry copper-molybdenum deposits, Valley, Lomex, Bethlehem, Highmont and JA, located within a fifteen square kilometre area in Highland Valley in the central part of the late Upper Triassic Guichon Creek batholith. The batholith, a composite, calc-alkaline and !-type intrusion emplaced about 210 million years ago, cuts and metamorphoses the Carnian to Norian (230 Ma to 208 Ma) Nicola Group country rocks. Gravity surveys indicate that the intrusion is a flattened, funnel-shaped body with a centrally located, steeply plunging root zone under Highland Valley. The jive major deposits lie around the projection of this zone to surface.
The batholith is divided into segments by northerly and northwesterly striking faults that are intimately related to mineralization. Almost all sulphide mineralization in Highland Valley deposits is in fractures, veins, faults or breccias. Fracture density is the most important single factor influencing ore grades, although mineralized breccias and faults are important at some deposits. Mineralization (dates range from 192 Ma to 202 Ma) is slightly younger than the 210 Ma age of crystallization of the batholith. The first mineralizing event in the batholith followed emplacement of the intermediate-aged Bethlehem phase, and produced the Bethlehem orebodies, Krain, South Seas (Trojan) and several other smaller deposits. The second, and most significant mineralizing event, followed emplacement of the youngest major phase of the batholith, the Bethsaida. At this time Valley, Lomex, Highmont, JA and several smaller deposits developed. Regional smle, green sericite vein and chlorite (epidote) vein alteration types are widespread in the centre of the batholith, and both types are roughly centred around and grade outward from propylitic alteration zones enclosing the Valley, Lomex, Bethlehem, Highmont and JA deposits. As well, geochemical studies show anomalous sulphur and copper concentration haloes up to 500 m wide around the deposits.
This paper emphasizes features of the Valley deposit, which occurs in an area of intense fracturing near the intersection of the northerly-trending Lomex Fault and the northwest- to westerly trending Highland Valley Fault. Predominant orientations of faults, fractures and quartz veinlets in the deposit parallel these two regional trends.
In general, proximal alteration in Highland Valley deposits consists of central silicic and potassic alteration, central to intermediate, overlapping and partly overprinting phyllic and argillic alteration, and fringing propylitic alteration zones. Copper ore is commonly best developed in the phyllic zone but extends locally into the silicic silicic, potassic and argillic zones. At Valley, a major zone of potassic feldspar alteration in the west-central, deeper part of the deposit is intimately associated with, and enveloped by an extensive zone of moderate to strong phyllic and pervasive argillic (pervasive kaolinite and sericite) alteration. These zones grade outward into a zone dominated by weak to moderate argillic alteration fringed by a mixed zone of weak to moderate propylitic alteration containing areas with minimal hydrothermal alteration. A well developed zone of silicic alteration, and an intense stockwork of largely barren quartz veinlets, the silicic re-entrant, occurs in the southeastern part of the deposit. Elsewhere in the deposit quartz veinlets are generally mineralized, but only moderately developed. Copper grades are generally highest in the phyllic and pervasive argillic alteration zones. Principal hydrothermal metallic minerals in Highland Valley deposits are bornite, chalcopyrite, pyrite and molybdenite. Locally, specular hematite, magnetite and chalcocite are important. Sphalerite, galena, tetrahedrite, PY"hotite, enargite and cove/lite occur in trace amounts. Metallic mineral zoning is well developed. The pattern at Valley is typical, with central bornite through chalcopyrite to fringing pyrite-dominated zones, albeit in the case of the Valley deposit the pyrite content is less than 1%. The Valley deposit, as defined by the 0.36% Cu isopleth, is oval in plan, 1000 m by 1300 m, with a broad halo of lower copper grades around it. Molybdenum and zinc form annular, geochemically-enriched zones around the deposit. Molybdenum is at trace levels at Bethlehem, and barely recoverable at Valley, but is significant in the Lomex, Highmont and JA deposits. Supergene enrichment is not an important factor in major Highland Valley deposits, but is important north of Highland Valley at the Krain deposit (Christie, 1976). An oxide zone developed at Valley, for example, generally averages 4.5 m in thickness but varies from 0.3 m to as much as 98 m in the southeastern comer of the deposit. At Valley, there is generally a slight depletion in copper grades in the oxidized zones, and, locally, a spotty, 3 m to 6 m thick supergene zone is developed just below the oxidized zone. Various exploration techniques were tried on Highland Valley deposits, but only Induced Po/ariQJtion (l.P.) was consistently useful in locating significant copper-molybdenum mineralization. Such surveys outlined large, moderate intensity chargeabi/ity anomalies over the Valley, Lornex and Bethsaida deposits, and a weak but distinct chargeability anomaly over the Highmont deposits. An J.P. survey by ASARCO Inc., in about 1956, outlined a moderate chargeability anomaly over the area of Bethlehem's Jersey and East Jersey deposits. Data for the JA area are not available.
In the Highland Valley district, only the Valley and Lornex deposits are presently in production. Remaining mineable reserves on January 1, 1994 were 507.8 million tonnes at 0.44% Cu and 0.006% Mo in the Valley deposit, and 119.2 million tonnes at 0.36% Cu and 0.013% Mo in the Lornex deposit, for combined reserves of 627 million tonnes at 0.42% Cu and 0.007% Mo. However, neither deposit is completely drilled off; both have further reserve potential at depth. Former producers, Bethlehem and Highmont, are now closed; JA remains undeveloped and uneconomic.