Metamorphism and gold mineralization in the Blue Ridge, southernmost Appalachians

Lode gold mineralization in the Blue Ridge of the southernmost Appalachians is hosted by metavolcanic rocks (e.g., Anna Howe mine, AL; Royal Vindicator mine, CA), metaplutonic rocks (e.g., Hog Mountain mine, AL), and metasedimentary rocks (e.g., Lowe, Tallapoosa, and Jones Vein mines, AL). Most gold occurs in synkinematic quartz +/- plagioclase +/- pyrite +/- pyrrhotite +/- chlorite veins localized along polydeformational faults that juxtapose rocks with significantly different peak metamorphic mineral assemblages. Mineralogy, chemistry, and O and H isotope studies suggest that the three types of host rocks have undergone differing amounts and types of alteration during mineralization. Limited wall-rock alteration in metavolcanic- and metasediment-hosted deposits, and relatively extensive wall-rock alteration in granitoid-hosted deposits, suggests that most deposits formed from fluids that were close to equilibrium with metavolcanic and metasedimentary rocks. Stable isotope compositions of the fluids calculated from vein minerals and vein selvages are consistent with a predominantly metasedimentary fluid source, but vary from deposit to deposit (-22 to -47 parts per thousand delta D, 4-5 parts per thousand delta(18)O, and 5-7 parts per thousand delta(34)S at Anna Howe and Royal Vindicator; -48 to -50 parts per thousand delta D, 9-13 parts per thousand delta(18)O, and ca. 19 parts per thousand delta(34)S at Lowe and Jones Vein; and -22 to -23 parts per thousand delta D, 8-11 parts per thousand delta(18)O, 9-10 parts per thousand delta(34)S, and -6 delta(13)C at Hog Mountain). Silicate mineral thermobarometry of vein, win selvage, and wall-rock mineral assemblages indicate that mineralization and regional metamorphism occured at greenschist to amphibolite facies (480 degrees +/- 75 degrees C at Anna Howe, 535 degrees +/- 50 degrees C at 6.4 +/- 1 kbars at Lowe, 530 degrees +/- 50 degrees C at 6.9 +/- 1 kbars at Tallapoosa, and 460 degrees +/- 50 degrees C at 5.5 +/- 1 kbars at Hog Mountain). Oxygen isotope fractionation between vein minerals and selvage minerals consistently records equilibration temperatures that are similar to or slightly lower than those estimated from silicate thermometry. Auriferous veins contain numerous fluid inclusions that were emplaced in several stages and can be subdivided into five compositional types based on salt and CO2 concentrations. Fluid inclusion isochores for early formed inclusions from these veins intercept the pressure and temperature conditions estimated from silicate mineral thermobarometry and stable isotope thermometry, and are compatible with entrapment at those conditions. These fluids exhibit significant variation in salinity (X(NaClequiv) = 0.0-0.2) and CO2 (X(CO2) = 0.0-0.2), suggesting variation in fluid-wall-rock interaction that accompanied gold deposition during declining temperatures. Less abundant and later fluids within the veins are dominantly CO2. The association of gold mineralization with structurally controlled concordant and discordant quartz sulfide veins, and the temperatures and pressures of wall-rock alteration and regional metamorphism indicate that the present distribution of gold is a result of metamorphism during progressive D-2-D-3 deformation. Isotopic data for alteration envelopes date this event as Alleghanian: 279 +/- 14 Ma (K-Ar whole rock) and 343 +/- 18 Ma (K-Ar biotite) at Lowe; and 315 +/- 18 Ma (Rb-Sr whole-rock isochron; Sr-87/Sr-86(i) = 0.7061 +/- 0.0008) and 294 +/- 16 Ma (K-Ar whole-rock) at Hog Mountain. Available data are compatible with development of the lodes during early. Alleghanian overthrusting of allochthons over sedimentary rocks of the autochthonous North American margin. The implication is that the fluids were derived from metasedimentary and/or metavolcanic formations in the lower parts of the crystalline thrust stack (or possibly from underlying autochthonous sedimentary formations), ascended along permeable fault zones, and were emplaced as veins into dilatent areas in and adjacent to the fault zones.