Significant gold occurrences located in proximity to Devonian granitoid intrusions have been recently discovered in the Clarence Stream area of southwestern New Brunswick. The area straddles the Ordovician St. Croix Terrane and Silurian Mascarene Belt, which are variously affected by four phases of regional deformation. The St. David antiform is interpreted as a megascopic F2-F3 fold-interference structure, the southeastern limb of which is cut by the late-D3, dextral Sawyer Brook Fault. Mineralized quartz veins and breccia units within the Ordovician Cookson Group are structurally associated with thrust-related D2 high-strain zones and folds. Southward, the occurrence of mineralized quartz veins in the Silurian Waweig Formation is influenced by the competency of gabbroic bodies, whereas late-D3 high-strain zones control their present geometry. The distinct lithostratigraphic assemblages and the nature and location of structures hosting the gold mineralization indicate that exploration strategies should not be restricted only to the proximity of specific intrusions or shear zones.

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 40 Ar/ 39 Ar 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 H 2 O, salts, and CO 2 with variable amounts of N 2 and CH 4 ; 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 δ 18 OSMOW and δ 13 CPDB values of mineralization-related ankerite range from +14.5‰ to +16.5‰, and -6.8‰ to -8.3‰, respectively. The δ 18 O 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.Sommaire -L'indice aurifère de Poplar Mountain dans l'ouest du Nouveau Brunswick est contenu dans le complexe volcanique de Poplar Mountain (CVPM), lequel est situé le long du segment sud de la zone de faille régionale de Woodstock. Les trois unités principales du CVPM sont, de la base au sommet, une unité volcanique felsique porphyrique, une unité volcaniclastique et une unité volcanique mafique. La datati...

The Poplar Mountain Volcanic Complex consists of three principal volcanic and subvolcanic units in the Poplar Mountain area, i.e. porphyritic dacite, dacitic volcaniclastic rocks, and basaltic volcanic rocks. The porphyritic dacite is a subvolcanic intrusion located in the centre of a volcanic dome and is sur-rounded by dacitic volcaniclastic rocks. The basaltic unit is stratigraphically above the felsic units. Gold mineralization is mainly associated with arsenopyrite, and to a lesser extent with pyrite and stibnite. The sulphides are disseminated in the dacite, and in various quartz-carbonate-sericite veins and quartz-cemented breccia. The mineralized zones are characterized by a higher degree of fracturing and veining than the barren zones, and are closely related to phyllic (sericite+carbonate+quartz+illite) alteration. Propyllitic (chlorite+carbonate+illite) alteration is widespread, and is mainly preserved in nonmineralized zones. The mineralization and alteration share some analogies with low-sulphidation epithermal deposits, or the outer zone of a porphyry-type system.

Gold-bearing mineralized zones in Ordovician turbidites of the Kendall Mountain Formation (St. Croix Terrane) in southwestern New Brunswick occur within shallowly dipping brittle high-strain zones. The gold zones, commonly several metres wide, are defined by stockwork, massive, and multiply brecciated quartz-sulphide veins containing arsenopyrite, pyrrhotite, pyrite and stibnite, and are enveloped by locally auriferous, weakly to intensely altered wall-rock. Vein paragenesis includes: 1) early (barren?) quartz-chlorite veins that are largely the product of migration of quartz from domains of cleavage-parallel pressure solution into extensional structures; 2) cross-cutting, multiphase, vuggy quartz-sulphide veins that contain most of the gold and are interpreted to be controlled by late-D2 thrust faults; 3) late cross-cutting, non-gold-bearing veins containing various combinations of laumontite, chlorite, muscovite, quartz, fluorite and base-metal sulphides. These late veins constrain the upper time limit of gold mineralization to Late Devonian based on their widespread association with nearby plutons of that age. Alteration associated with gold is lithology-specific and notably carbonate-poor and appears to reflect overprinting of several events. Major elements removed during alteration include Si, Na, Mn, Mg; minor and trace elements include Co, Li, Sc and possibly Sr, Ni and Zn. Minor and trace elements added include Au, Ag, As, Sb, S, and possibly F, Cu, Pb and Zr. The gold-mineralized zones appear to be the product of a protracted structural history that culminated with late-D2 thrust-faulting, and which facilitated gold mineralization. The gold deposits of Anomaly A are tentatively interpreted to be affiliated with intrusion-related gold deposits in the Clarence Stream Main Zone area.