Abstract
The Middle Jurassic A6 Anomaly is located 30 km southeast of Eskay Creek, north-central British Columbia and consists of thick, altered felsic igneous rocks overlain by a mafic volcano-sedimentary package. Lithogeochemistry on igneous rocks, X-ray diffraction on altered felsic units, and electron probe microanalysis and secondary ion mass spectrometry on illite and quartz were applied to explore the volcanogenic massive sulfide (VMS) potential, characterize alteration, and determine fluid conditions at the A6 Anomaly. Lithogeochemistry revealed calc-alkaline rhyodacite to trachyte of predominantly FII type, tholeiitic basalts with Nb/Yb < 1.6 (i.e., Group A), and transitional to calc-alkaline basalts and andesites with Nb/Yb > 2.2 (i.e., Group B). The felsic units showed weakly to moderately phyllic alteration (quartz–illite with minor orthoclase and trace chlorite–pyrite–calcite–barite–rutile). Illite ranged in composition from illite/smectite (K = 0.5–0.69 apfu) to almost endmember illite (K = 0.69–0.8 apfu), and formed from feldspar destruction by mildly acidic, relatively low temperature, oxidized hydrothermal fluids. The average δ18O composition was 10.7 ± 3.0‰ and 13.4 ± 1.3‰ relative to Vienna Standard Mean Ocean Water for illite and quartz, respectively. Geothermometry involving illite composition and oxygen isotope composition on illite and quartz yielded average fluid temperatures of predominantly 200–250 °C. Lithogeochemical results showed that the A6 Anomaly occurred in a late-Early to Middle Jurassic evolving back-arc basin, further east then previously recognized and in which transitional to calc-alkaline units formed by crustal assimilation to enriched Mid-Ocean Ridge Basalt (EMORB) (i.e., felsic units, Group B), followed by thinning of the crust resulting in tholeiitic normalized MORB basalts (i.e., Group A) with a minor crustal component. The alteration assemblage is representative of distal footwall alteration, and metal transport in this zone was limited despite favorable temperature, pH, and redox state, indicating a metal depleted source (i.e., felsic units).
Highlights
North-west British Columbia is a highly metal-endowed region, often colloquially dubbed the “Golden Triangle”
The results of whole rock lithogeochemistry for the 136 igneous and volcaniclastic rocks from the A6 Anomaly are shown in Figures 7–12, Table 2 and Supplementary Materials Table S4
Alkali earth (CaO) and alkali (Na2O, K2O) elements show a wider compositional range due to their mobility related to hydrothermal alteration (Figure 8d,e)
Summary
North-west British Columbia is a highly metal-endowed region, often colloquially dubbed the “Golden Triangle”. Mineralization in the area is hosted predominantly within the Lower to Middle Jurassic Hazelton Group (e.g., [1,2,3]). Among the known deposits in the region, the majority are porphyry and epithermal deposits formed in a volcanic arc in the Lower Jurassic (e.g., [1,7,8]). Of particular interest for VMS exploration are lithologies in the Iskut River Formation of the Upper Hazelton Group, as these are interpreted to have formed, in part, in a late-Early to Middle Jurassic back arc rift which is a favorable VMS setting (e.g., [13,14,15,16,17]). Exploration efforts targeting VMS systems have been following the trend of this rift but with little success so far
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