"Rock of the Month # 253, posted for July 2022" ---

Molybdenite is molybdenum disulphide, MoS₂ - it is the only significant ore mineral of molybdenum. Secondary minerals of Mo do occur (e.g., oxides such as molybdite, ferrimolybdite, and powellite) but these are not economically significant. Additional Mo minerals are generally very rare: silicates, selenates, arsenates...

Molybdenite occurs in two polytypes, of hexagonal and trigonal crystal systems. It has a distinctive blue colour, combined with metallic lustre, and is often found as thin flakes, reminiscent of mica or graphite. Like graphite, molybdenite is very soft, and will leave marks on paper. Also like graphite, molybdenite is opaque, and strongly anisotropic when viewed under reflected, polarized light.

The sample is from the old Boss Mountain mine, northeast of 100 Mile House in the Cariboo district of central British Columbia. A critical suite of mineral deposits of copper and other metals are referred to as porphyry deposits, an allusion to the coarse crystals (feldspar phenocrysts) seen in the associated igneous rocks. These rocks are commonly of felsic to intermediate composition, whether intrusive (e.g., granodiorite, diorite) or extrusive, volcanic rocks (e.g., andesite). The ore minerals are generally disseminated, the ore of modest grade. Thus porphyries, though they may have rich marginal deposits (including skarns), tend to be large in tonnage and low in grade. For this reason, the typical porphyry deposit is worked as an open pit mine, not underground. The most common subset of economically -important porphyries are known as porphyry copper deposits (with copper, and often some gold and/or molybdenum). Porphyry molybdenum deposits are less common but critical for their principal metal. Examples are Boss Mountain and Endako (BC), Setting Net Lake (Ontario), Climax and Henderson (Colorado) and Quartz Hill, Alaska (Kirkham and Sinclair, 1995; McMillan et al., 1995). Other porphyries are enriched in tungsten and molybdenum, or in tin, and may carry other metals such as silver or platinum group elements.

In the Boss Mountain area, gold prospectors first found a molybdenite showing in 1911, while panning on Molybdenite Creek. Endako and Boss Mountain (also known as Takomkane Mountain) were recognized as porphyry molybdenum deposits with economic potential (BCMEMPR, 1963, pp.30-41; Vokes, 1963, pp.246-256). Boss Mountain was one of a number of mines that opened with the help of Vancouver-born mining engineer Bernard Brynelson (Anon, 2004). It was developed as an underground mine accessed by adit, and production began in February 1965. Mine production was suspended in 1973-1974, and terminated in 1984, due to the economics of the day. The average grade realised in 1965-1971 was 0.26% Mo.

Soregaroli (1968) offered a detailed description of the deposit. Mineralization was found in a cirque at the head of Molybdenite Creek, outcropping at the 5400-5700' (1646 to 1737 metres) altitude, on the N.E. slope of Takomkane (Big Timothy) Mountain. The deposit is located where the Boss Mountain quartz monzonite **(porphyry) stock intrudes granodiorite of the Takomkane batholith. Soregaroli identified four stages of hydrothermal alteration and five periods of fracture development, pertinent to the "preparation" of initially fresh igneous rocks to localise the Mo mineralization.

The role of magmatic fluids, and development of host structures, were considered in detail by Macdonald (1983). This later study found that the Boss Mountain stock ** was technically an adamellite or monzogranite (in QAP proportions, with 25-60 volume percent Quartz, and subequal amounts of orthoclase [Alkali feldspar] and Plagioclase feldspar). The Mo mineralization is associated with the stock, and postdates the larger host batholith. The Mo occurs in breccia pipes and a sheeted vein complex.

The Boss Mountain deposit is dated at circa 102 Ma (mid-Cretaceous), and Endako at 145 Ma (earliest Cretaceous: Tom Schroeter in Northern Miner, 2007). The milling of Mo ores and other porphyry-type ores in Canada was reviewed in Pickett (1978), including mines such as Brenda, Endako, Lornex, Highland Valley, Boss Mountain (ibid., pp.320-323) and Island Copper.

The principal importance of molybdenite is as molybdenum ore. The platy habit, colour, lustre and (in some cases) hexagonal outline of the flakes can generate attractive mineral specimens. Because of the perfect, mica-like basal cleavage, molybdenite has been used as a clean, flat, relatively high atomic-number (Mo = 42) specimen substrate for electron microscopy.