Gold skarn mineralization and fluid evolution in the Nickel Plate Deposit, British Columbia

Abstract

The Nickel Plate orebody in the Hedley district is the largest gold skarn deposit in Canada. During the period 1904 to 1955 it produced almost 49 million metric tons (Mt) of gold representing approximately half of the gold produced from all skarns in British Columbia. The deposit is hosted by thinly bedded calcareous and tuffaceous siltstones and limestones of the Upper Triassic Nicola Group, an accreted oceanic-arc assemblage. These rocks are intruded by numerous sills, dikes, and small stocks of subalkalic, calc-alkaline quartz diorite, diorite, and gabbro. Skarn alteration and related gold mineralization overprints sedimentary and igneous rock and is zoned about the intrusive contacts.The early stages of alteration are marked by the formation of biotite, potassium feldspar, and quartz hornfels in siltstone and by potassic alteration of intrusive rock. The biotite contains <0.55 mole fraction phlogopite (Mg/(Mg + Fe)) and <2 wt percent TiO 2 which are features indicative of the reduced nature of this mineralization compared to that of porphyry Cu systems. This early stage of alteration is temporally continuous into formation of pyroxene skarnoid which consists of intermediate pyroxene (Hd (sub 48-59) ) in siltstone and Mg-rich pyroxene (Hd (sub 5-30) ) and K feldspar in intrusive rocks. The most volumetrically important alteration style, and the primary host to gold mineralization, consists of dark green, fine-grained pyroxene (Hd (sub 60-94) ) and lesser grandite garnet (Ad (sub 14-82) , avg Ad 35 ) which overprints earlier hornfels and skarnoid, and often completely replaces both sedimentary and igneous protoliths. Ore-bearing sulfides and scapolite occur with the hedenbergite skarn and consist of pyrrhotite, arsenopyrite with inclusions of gold, hedleyite, native bismuth, and minor chalcopyrite, sphalerite, and galena. Retrograde alteration in the deposit is limited, with only minor to trace amounts of wollastonite, prehnite, epidote, and chlorite replacing garnet and pyroxene.Fluid inclusion homogenization temperatures indicate that the main body of pyroxene-garnet skarn formed at an average temperature (pressure corrected) between 460 degrees and 480 degrees C with locally high temperatures of 700 degrees to 800 degrees C. Fluid salinities average 18.3 and 9.7 wt percent NaCl equiv for garnet and pyroxene, respectively. Dissolution temperatures of halite daughters in pyroxene and quartz from skarn indicate a maximum fluid salinity of 37.9 wt percent NaCl equiv. Homogenization temperatures in scapolite associated with sulfide mineralization indicate a lower temperature for this mineralization in the range 320 degrees to 400 degrees C.Estimated physiochemical conditions of formation based on the composition and stability fields of major calc-silicate and sulfide minerals indicate that the hedenbergite skarn, pyrrhotite, and arsenopyrite were deposited under similar conditions of sulfur fugacity (log f (sub S 2 ) = -10 to -3) and temperature (400 degrees -600 degrees C), whereas a log f (sub O 2 ) = -25 to -22 is estimated for the hedenbergite skarn.Spatial trends in pyroxene and garnet composition and the distribution of fluid inclusion homogenization temperatures indicate that the eastern contact of the Toronto stock and the locus of diorite dikes, such as the South dike, in the ore zone acted as critical conduits for channeling of hydrothermal fluids. These two separate fluid paths may have resulted in the two different compositional trends found in the pyroxene and garnet from locations in the ore zone versus those more proximal to the Toronto stock.