Preliminary Map 20 describes the geology of the Morehead Lake area (93A/12) in south central British Columbia. The Preliminary Map consists of a geology map at 1:31 250 scale and accompanying notes.
The lowermost rocks in the area are argillite, sandstone, conglomerate and limestone of possible Norian age. Overlying is unit 2, which consists of green and maroon basaltic andesites, which have pyroxene with or without hornblende phenocrysts. Flows, volcanic breccias, lahars, pillow lavas and pillow breccias occur. Above unit 2b, nearly all the rocks are maroon. Rocks of unit 2c tend to be highly amygdaloidal. Units 2d to 2e are basaltic with analcite phenocrysts. Polylithic laharic breccia comprises unit 2f. Unit 2g consists of pyroxene basaltic andesite or hornblende-pyroxene andesite. Unit 2h pyroxene basaltic andesites occur as dikes or feeders for 2g. The style of volcanism changed and unit 3 is characterized by the presence of felsic clastic material of trachytic, latitic, syenitic, and monzonitic texture and composition. Polylithic laharic breccias are typical. Sedimentary interlayers in unit 3 are limestone, immature sandstone, conglomerate, and mudstone. It may be of Hettangian age. A series of monolithic felsic breccias, brecciated syenite plugs and tuffs may represent eroded felsic dome complexes. Unit 4 is similar to the maroon basaltic andesites of unit 2, except that it is analcite-rich and plagioclase is rare. Unit 5 comprises quartzofeldspathic sandstone and grit. It is Cretaceous or younger.
Intrusive rocks form three main groups. The oldest are medium to fine-grained monzonites and diorites. Somewhat younger are medium- to fine grained syenites and monzonites that comprise the bulk of the Mount Polley stock. These returned a K-Ar age of 184±5 m.y. and intrude Hettangian Weyla-bearing strata. The Bootjack stock is nepheline syenite. It is likely Lower Jurassic or younger. There are also possible Cretaceous intrusions, mainly quartz monzonite dikes and monzonite plugs that cut unit 4 rocks.
Block faulting is the dominant structural style of the map-area. Northeasterly striking faults are the most common. Most dip steeply.
Five kinds of copper deposits have been recognized in the area. These are: copper associated with amygdaloidal basalts; stratiform deposits, chalcocite in felsic sandstone; alkalic porphyry deposits, like the Cariboo Bell (later called the Mount Polley) deposit, which is associated with syenites and monzonites; copper in hornblende diorite and monzonite, where copper minerals occur in pyrite zones; and copper and molybdenum in quartz monzonite. Alluvial gravels along the Cariboo and Quesnel rivers also contain some placer gold.