Hydrothermal alteration in anthracite from eastern Pennsylvania: Implications for mechanisms of anthracite formation

Orthogonal joint sets (cleat) in anthracite-rank coal beds from eastern Pennsylvania contain two mineralogically and chemically different authigenic clay mineral assemblages. In localities from all four anthracite fields, the systematic cleat contains significant quantities (20 to 95 wt%) of authigenic rectorite, sudoite, or tosudite, which are clay minerals primarily associated with hydrothermal veins and ore deposits. The strike of systematic cleat in this region roughly parallels the inferred direction of lateral compressive stress during the Alleghany orogeny. The opposing nonsystematic cleat contains mostly authigenic NH ₄ -illite, a clay that also occurs in the coal matrix. All of these authigenic clay minerals formed during anthracitization (T> 200 °C) by replacement of kaolinite and quartz, both of which formed during an earlier stage of coalification and are present in the joints and matrix of nearly all coal samples. These distinct mineralogical-structural relations are observed throughout the Anthracite region and suggest that (1) mineralogical variations in the coal joints are related primarily to permeability variations that were controlled by the orientation of Alleghanian stress fields, (2) hydrothermal fluids passed through the systematic cleat during anthracitization, and (3) hydrothermal alteration influenced diagenesis in this region. Hydrothermal alteration may be related to basin-wide fluid migrations that were driven by Alleghanian-age uplift. Basinal fluid flow, concentrated along permeable joints and detachment zones, could have efficiently transported heat from depth and thereby increased the rate of coalification and decreased the postulated minimum depths at which anthracitization occurred (perhaps ≤5km vs. 6-10 km).