|Author||Anderson, R G; Schiarriza, P; Andrews, G; Breitsprecher, K; Davis, W; Dunn, C E; Plouffe, A; Thomas, M D|
|Abstract||The composite Late Triassic-Early Jurassic Thuya Batholith, prospective for base and precious metals, occurs in the southern Nicola Arc (Quesnel Trough) of Quesnellia, between well-known porphyry
districts at Highland Valley (Cu-Mo) and Afton-Ajax (Cu-Au) to the south and Gibraltar (Cu-Mo) and Mt. Polley (Cu-Au) to the north. Limited mapping and low-precision K-Ar and U-Pb isotopic dates had suggested a correlation with the latest Triassic
Guichon Creek suite, and, by inference, calc-alkaline porphyry Cu-Mo potential. |
Assessing the mineral potential of the batholith is challenging due to limited exposure, and shallow burial beneath significant Tertiary volcanic and Quaternary
glacial deposits. Multiple approaches are required, therefore, to evaluate the batholith's mineral potential, including: bedrock and surficial mapping; physical volcanology; high resolution geophysics; till geochemistry; and biogeochemical studies
of pine and spruce.
Recent bedrock mapping of and high-precision U-Pb dates for the Thuya Batholith (Figure 1) indicated a complex, multi-episodic intrusion of heterogeneous and homogeneous plutonic phases into Upper Triassic (Carnian) Nicola
Group clinopyroxene-bearing volcaniclastic and sedimentary rocks (e.g., Schiarriza et al., 2002). The country rocks were deformed, metamorphosed and locally mineralized during intrusion (Figure 2).
The batholith includes the latest Triassic (<202
Ma to >198 Ma) Rayfield River phase in the west, the Early Jurassic (196-193 Ma) Eakin Creek suite in the east, and medial Middle Jurassic (164-161 Ma) Bonaparte Lake phase (Figure 3). The oldest phases and suites represent members of the regional
Copper Mountain (ca. 206-200 Ma; with significant alkaline porphyry Cu-Au-Ag-mineralization) and Early Jurassic Wildhorse plutonic suites as recently defined (see Breitsprecher et al., this volume).
The latest Triassic Rayfield River phase,
comprising hornblende-biotite syenite, is the host for the Cu-Au gold deposit of the same name. In the Rayfield River area, north- and east-trending brittle faults localize some alteration and base-metal mineralization and apparently were
remobilized to localize Neogene, basanite and nephelinite centres which contain mantle xenoliths.
Eakin Creek suite comprises biotite-hornblende diorite and quartz monzodiorite which grades radially inwards to quartz monzonite and alkali feldspar
megacrystic monzogranite phases. Propyllitic alteration is widespread and characteristic. The mafic rocks along the northern batholithic margin and in satellitic stocks to the north are associated with many of the base metal Cu-rich showings (Fig.
4) Ultramafic and minor syenitic rocks (Dum Lake suite) occur mainly along the north-eastern flank of the batholith and host Au vein and PGE occurrences.
The younger, higher level, unaltered, felsic and apparently unmineralized biotite
monzogranite of the Middle Jurassic Bonaparte Lake phase (ca. 163-161 Ma) underlies much of the western and central areas, near Bonaparte Lake.
The composite Mt. Hagen stock along the south-eastern flank of the batholith is mainly underlain by
texturally heterogeneous biotite syenite phases which intruded micro-diorite. A bladed, alkali feldspar porphyry pegmatite variant of the syenite phase hosts copper-gold showings near the summit of Mt. Hagen.
A variety of geochemical compositions
and affinities are represented in the batholith. Rayfield River and Mt Hagen syenites are alike and alkaline, feldspathoid-normative, and very high-K (shoshonitic). The Early Jurassic Eakin Creek suite ranges from subalkaline to alkaline and is
generally high-K; partial overlaps and changes in compositional trends for the mafic and felsic phases are typical of a fractionated suite. Middle Jurassic Bonaparte Lake phase is subalkaline, calc-alkaline, and average in potash affinities. All
units are metaluminous, lack Eu anomlies in the chondrite-normalized rare earth patterns and have volcanic arc granite trace element compositional affinities.
Miocene Chilcotin Group basalt flow rocks cover the mineralized basement rocks but
improved models of landscape control on lava facies distribution, and assessment of existing and new geological and geophysical survey data help minimize their well-known impediments to mineral exploration (see Dohaney et al., (2010) and Andrews et
al., this volume)..
High resolution aeromagnetic and radiometric maps (see Thomas and Pilkington (2008) and Thomas et al., this volume) refine distribution of host and cover volcanic and granitoid rocks which locally have a strong correlation
with aeromagnetic patterns. Other derivative magnetic maps identify cryptic faults.
Surficial mapping and mineralogical and geochemical analyses of till samples (see Plouffe et al., 2009, 2010 and this volume) indicate that two glacial ice
movements resulted in dispersal of gold and base metal minerals and geochemical anomalies to the west and south of their source areas.
More than 500 chemical analyses of outer bark samples from spruce and pine help discern compositionally-distinct
bedrock substrate, provide focus for follow up work in areas yielding anomalous levels of metals absorbed by the trees, and possibly identify concealed mineralization (see Dunn et al., this volume).
These studies combine to suggest a focus for
base and precious metal exploration along the northern and western margins of the Thuya Batholith, specifically targeting the Eakin Creek mafic phase diorite and Rayfield River and Mt. Hagen syenite phases.