Abstract

The Jean Marie in the July Lake area of northeastern British Columbia consists of mainly limestones and partially dolomitized limestones, with variable dolomite content and porosity of dissolutional origin. Pure dolostones occur as metre-thick bodies, at the very top of the Jean Marie in only a few wells. Dolomite also occurs as a selective replacement of internal sediment infilling fractures. This mode of occurrence, as well as the patchy distribution of pure dolostones at the very top of the Jean Marie, suggests that matrix replacement dolomites formed from fluids that were delivered from flow through a vertical, or near-vertical, fault-conduit system, rather than from fluids that were delivered from flow laterally through matrix pores in a stratal-controlled system. Later mineral phases, including (in paragenetic order) saddle dolomite cement, sphalerite, dickite and barite, are also interpreted as part of the fault/fracture controlled system of diagenesis. The homogenization temperatures of saddle dolomite range from 80°C to 163°C and those of sphalerite and barite, collectively, from 55°C to 126°C, with a broad range of calculated salinities for all these phases ranging from approximately 9.0 to 27.0 wt.% NaCl + CaCl2 equivalent. The wide range of salinities for these phases is interpreted to be from the mixing of evaporative brines with less saline formation waters away from the fault/fracture conduits.

The δ13C and δ18O values for a combined set of matrix replacement dolomites and saddle dolomites fall along a covariant linear trend, suggesting both phases formed from fluids with similar isotopic compositions. Using the oxygen isotopic composition calculated for fluids from which saddle dolomite formed (+0.7‰ SMOW), crystallization temperatures for matrix replacement dolomites of approximately 80°C to 100°C were calculated from the range of δ18O values. The range of crystallization temperatures for both matrix replacement dolomites and saddle dolomites far exceeds the ambient formation temperature at the time of dolomitization or dolomite cementation, estimated to be no higher than 50°C to 60°C. Thus, the formation of both matrix replacement dolomites and saddle dolomite cements conforms to the standard for inferring a hydrothermal origin. The estimated range of ambient formation temperatures suggests an interval of hydrothermal diagenesis possibly starting as early as latest Devonian and continuing into the early Mississippian, at burial depths no deeper than approximately 900 m.

The parent fluids for the hydrothermal phase are interpreted to be residual evaporitic brines. During latest Devonian to early Mississippian time, heated brines flowed up through fault-controlled conduits, bypassing approximately 850 m of overlying shale and basinal limestone, into the Jean Marie Member until the flow was impeded by the overlying impermeable shales of the informal upper member of the Redknife Formation. The brines then moved laterally into permeable limestones with grain-support matrices of the upper transgressive-regressive (T-R) cycle of the Jean Marie. This resulted in the formation of scattered metre-thick dolostones at the top of the Jean Marie, and the partial dolomitization of limestones away from the faults.

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