THE STRATIGRAPHY OF THE NICOLA GROUP IN THE HEDLEY DISTRICT, BRITISH-COLUMBIA, AND THE CHEMISTRY OF ITS INTRUSIONS AND AU SKARNS

The Nicola Group at Hedley, British Columbia, is a late Carnian to lat e Norian (Late Triassic) sequence of calcareous sedimentary and are-re lated volcaniclastic rocks. It was deposited on a tectonically active paleoslope that marked either the rifted eastern margin of the shallow -marine Nicola basin or the faulted edge of an intrabasinal platform. The lower part of the Nicola Group comprises a succession of four esse ntially coeval sedimentary facies. From east to west across the distri ct, these are informally named the thin (approx. 200 m), shallow-marin e, limestone-dominant French Mine formation; the thicker, calcareous s iltstone-dominant Hedley and Chuchuwayha formations in the central par t of the district; and the thick (up to 2200 m), deeper water and argi llite-dominant Stemwinder formation. These facies are all blanketed by the Whistle formation, a 1200 m thick unit of basaltic tuff and tuffa ceous sediment whose base is marked by a gravity-slide megabreccia, th e Copperfield breccia. The Nicola are at Hedley was associated with tw o plutonic episodes. Oldest are the Hedley intrusions, which are relat ed to economic Au skarns, including the Nickel Plate deposit, which ha s produced over 71 t of gold from 13.4 Mt of ore. The Hedley intrusion s are similar in composition (quartz gabbro to quartz diorite) and ove rall metaluminous chemistry to other island-are-generated plutons rela ted to many Cu and Fe skarns in British Columbia, although they are le ss evolved. They also differ in having lower Fe2O3/FeO ratios (avg. 0. 23), indicating a reduced oxidation state, and higher Ba/La and Sc/Nb ratios. A slightly younger plutonic episode produced the 193 Ma (Early Jurassic) Bromley batholith and the 194 Ma Mount Riordan stock; the l atter is associated with the Mount Riordan (Crystal Peak) industrial g arnet skarn. Gold skams are preferentially developed in areas where th e Hedley intrusions cut the Hedley and French Mine formations. The Au skarn ore is marked by anomalous As, Bi, Te, and Co values, and by hig h pyrrhotite/pyrite and pyroxene/garnet ratios. It is distinct from th e ore of Fe, Cu, Mo, Pb-Zn, W, and Sn skams by its very low Cu/Au, Zn/ Au, and Ag/Au ratios (avg. 97, 18, and 12, respectively).