Chemostratigraphy and hydrothermal alteration of the Flat Landing Brook Formation, Brunswick Belt, Bathurst Mining Camp

Abstract

The Flat Landing Brook Formation, northern New Brunswick Bathurst Mining Camp, represents a Middle Ordovician supervolcano of tremendous volume. It consists of subaqueous to subaerial volcanic flows, domes, volcaniclastic deposits, and derived sedimentary rocks, mostly within a very narrow range of original calcalkalic/transitional A-type rhyodacite/rhyolite composition but variably obscured by hydrothermal alteration and multiphase deformation. The objective of the project was accomplished: to interpret the cryptic FLB rocks along the Brunswick Belt together with footwall Upper Nepisiguit Falls Formation and hanging wall Little River Formation via: (a) chemostratigraphy employing alteration-resistant and petrogenetically-sensitive ratios of low-mobility elements with contrasting compatibilities; and (b) alteration quantified from multiple precursor mass balance, alteration reaction vector, and mineral normative calculations. The sample suite from seven diamond-drill holes has whole-rock geochemical data (n = 346; XRF and INAA), core photographs (n = 89), Sm-Nd isotopes (n = 15), and O-isotopes (n = 36). Alumina-normalization of incompatible Zr versus compatible TiO2 (TiO2/Al2O3 vs. Zr/Al2O3) reveals emergent sample groupings, and profiled by downhole core sample depth serve as the basis for the chemostratigraphy. Each division is further classified by magmatic affinity (Zr/Y), alkalinity (Nb/Y), crustal/mantle melt (Th/Hf), Fe-Ti oxides stability (Ti/Ti*), REE enrichment/depletion ([La/Yb]cn, Eu/Eu*), and εNdt-derived crust/mantle melt ratios. The sixteen chemostratigraphic divisions are assigned: Nepisiguit Falls Formation felsic volcanic-derived sedimentary rock (NF SED; Zr/TiO2 = 0.059; Nb/Y = 0.28; Zr/Y = 3.72; Th/Hf = 2.17; Ti/Ti* = 0.072; [La/Yb]cn = 4.47; Eu/Eu* = 0.39) and Brunswick Horizon Member iron formation with tholeiitic andesite input (IF; Zr/TiO2 = 0.024; Nb/Y = 0.32; Zr/Y = 3.53; Th/Hf = 1.58; Ti/Ti* = 0.116; [La/Yb]cn = 6.70; Eu/Eu* = 1.17); Flat Landing Brook Formation effusive Reids Brook Member calc-alkalic rhyodacites (A1, A2, A3; Zr/TiO2 = 0.083, 0.085, 0.094; Nb/Y = 0.33, 0.39, 0.33; Zr/Y = 7.05, 8.10, 8.51; Th/Hf = 2.14, 1.48, 1.36; Ti/Ti* = 0.075, 0.093, 0.097; [La/Yb]cn = 7.54, 6.35, 5.33; Eu/Eu* = 0.47, 0.55, 0.61; εNdt = -3.42, -2.59, -2.20; crust/mantle = 64/36, 55/45, 57/43); younger, more voluminous effusive/explosive Roger Brook Member transitional/calc-alkalic rhyolite/rhyodacite (B1, B2, B3, B4; Zr/TiO2 = 0.153, 0.143, 0.116, 0.125; Nb/Y = 0.34, 0.33, 0.34, 0.34; Zr/Y = 6.04, 6.58, 7.09, 8.87; Th/Hf = 2.41, 1.86, 1.65, 1.21; Ti/Ti* = 0.034, 0.045, 0.063, 0.076; [La/Yb]cn = 7.57, 6.62, 6.05, 5.22; Eu/Eu* = 0.38, 0.48, 0.50 & 0.60); upper FLB unnamed flowbanded effusive rhyodacites (transitional C1 and tholeiitic C2; Zr/TiO2 = 0.089, 0.107; Nb/Y = 0.31, 0.32; Zr/Y = 5.73, 3.46; Th/Hf = 2.34, 3.18; Ti/Ti* = 0.059, 0.028; [La/Yb]cn = 6.95, 6.85; Eu/Eu* = 0.45, 0.37); Little River Formation unnamed tholeiitic andesite sedimentary rock (SED; Zr/TiO2 = 0.023; Nb/Y = 0.41; Zr/Y = 4.00; Th/Hf = 3.11; Ti/Ti* = 0.136; [La/Yb]cn = 8.15; Eu/Eu* = 0.63); unnamed felsic volcanic rocks marking transition into rift-stage (transitional/tholeiitic rhyolite C3 and feldspar porphyritic alkalic trachyandesite/rhyolite D1; Zr/TiO2 = 0.200, 0.153; Nb/Y = 0.38, 0.83; Zr/Y = 4.77, 6.98; Th/Hf = 2.08, 1.38; Ti/Ti* = 0.022, 0.039; [La/Yb]cn = 6.21, 7.51; Eu/Eu* = 0.42, 0.53); Brunswick Mines Member within-plate alkalic gabbro/basalt (ALK GAB; Zr/TiO2 = 0.012; Nb/Y = 0.78; Zr/Y = 6.38; Th/Hf = 0.99; Ti/Ti* = 0.521; [La/Yb]cn = 5.95; Eu/Eu* = 0.77); unnamed enriched mid-ocean ridge basalt (E-MORB) type tholeiitic gabbro (THOL GAB; Zr/TiO2 = 0.007; Nb/Y = 0.26; Zr/Y = 3.49; Th/Hf = 0.31; Ti/Ti* = 0.605; [La/Yb]cn = 1.70; Eu/Eu* = 0.96). A lithostratigraphic progression is observed from crustal toward more mantle melt petrogenetic compositions, extensional continental to rift basin tectonics, corroborated by Sm-Nd isotopes: Nepisiguit Falls Formation (εNdt=460Ma = -8.28 to -4.80; crust = 100 to 74%, mantle = 0 to 26%); Flat Landing Brook Formation (εNdt=460Ma = -4.80 to -1.40; crust = 74 to 49 %, mantle = 26 to 51 %); Little River Formation (εNdt=460Ma = -0.12 to -5.10; crust = 39 to 0 %; mantle = 61 to 100 %). Hydrothermal alteration is determined for each sample in chemostratigraphic hole profile to show the range of pervasive weak/moderate to layer selective intensities, for K-feldspathization/albitization (Na2O vs. K2O antipathy), sericitization/paragonitization (Na2O vs. K2O antipathy, SiO2 mobility), chloritization (gain of Fe2O3T and MgO, loss of alkalis and SiO2), silica leaching/flooding (SiO2 gain/loss), and calcite/ankerite/dolomite/siderite/magnesite alteration (gain of CaO +/- Fe2O3T and MgO). Within these profiled holes, there is no evidence of intense hydrothermal alteration, discordant stockwork zone, or significant massive sulfide accumulation in the Flat Landing Brook Formation. Applying similar chemostratigraphy and quantified alteration elsewhere in the Bathurst Mining Camp will help resolve complex lithostratigraphic relationships to focus mineral exploration.