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Geologic, Geochemical, and Geochronological Constraints on the Genesis of Gold Mineralization at Poplar Mountain, Western New Brunswick, Canada

G. Chi^1,, S. Watters^2,*, W.J. Davis³, P. Ni⁴, S. Castonguay⁵ and D. Hoy⁶

¹ Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, S4S 0A2.
² New Brunswick Department of Natural Resources, Geological Surveys Branch, 207 Picadilly Road, Sussex, New Brunswick, E4E 5L2.
³ Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario, K1A 0E8.
⁴ Department of Earth Sciences, Nanjing University, Nanjing, 210093 P. R. China.
⁵ Geological Survey of Canada, Québec Division, 490 rue de la Couronne, Québec, Québec, G1K 9A9.
⁶ Freewest Resources Canada Inc., 855 Field Street, Thunder Bay, Ontario, P7B 6B6.

Correspondence: Corresponding author. E-mail: guoxiang.chi{at}uregina.ca

The Poplar Mountain gold occurrence in western New Brunswick is hosted in the Poplar Mountain volcanic complex (PMVC), which is located along the southern segment of the regional Woodstock fault zone. The PMVC consists of three principal units including, in ascending order, a porphyritic felsic volcanic unit, a volcaniclastic unit, and a mafic volcanic unit. U–Pb dating of zircon indicates that the age of the volcanic rocks is younger than 459 ± 3 Ma, and ⁴⁰Ar/³⁹Ar dating of mineralization-associated sericite indicates that the age of mineralization is 411 ± 3.7 Ma. Gold-mineralized zones occur in all the lithologic units but mainly in the porphyritic felsic volcanic unit. Gold zones are not controlled by individual faults but are characterized by high-density brittle fracturing, thin carbonate-quartz veining, and intensive ankerite–sericite alteration, superimposed on an earlier chlorite–calcite –quartz alteration. Gold is associated with arsenopyrite, which mostly occurs as disseminations in the host rocks, and to a lesser extent with quartz–carbonate–sericite veins and quartz-cemented breccias.

Geochemical data indicate that mineralization-associated alteration is characterized by enrichment of K, Rb, Cs, Ca, Sr, Ba, As, Sb, W, C, and S. Fluid inclusion data indicate that the mineralizing fluids are composed of H₂O, salts, and CO₂ with variable amounts of N₂ and CH₄; salinities range from 2.3 to 10.6 eq. wt.% NaCl, but mainly from 2.3 to 5.6 eq. wt.% NaCl, and homogenization temperatures lie mainly between 220° and 270°C. Fluid pressures at the site of mineralization are estimated to have been 770 to 1200 bars, corresponding to a depth of 2.9 to 4.6 km at lithostatic pressure. The ¹⁸OSMOW and ¹³CPDB values of mineralization-related ankerite range from +14.5 to +16.5, and –6.8 to –8.3, respectively. The ¹⁸O values of the ore-forming fluids are estimated to be 6.4 to 8.3 at a temperature of 250°C. These carbon and oxygen isotope data fall in the field of magmatic fluids and in part the field of metamorphic fluids.

Considering the geological setting of the region, abundant granitic intrusions, the similarity between the age of mineralization (411 ± 3.7 Ma) and the nearby Pokiok batholith (402–415 Ma), and the enrichment of granophile elements in mineralization-related alteration, the ore-forming fluids were probably derived from a granitic intrusion underneath the PMVC. These fluids were focused along structures related to the Woodstock fault. The Poplar Mountain gold occurrence might be compared to some granitic intrusion-related gold systems based on the geochemical data presented herein.

Key Words: Gold deposits • intrusion-related gold systems • Poplar Mountain • Ar—Ar dating • fluid inclusions • CO₂ • isocon

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