Geochemical constraints on the origin of calcite veins and associated limestone alteration, Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A.

Abstract

Isotopic and elemental analyses constrain the origins of two types of calcite veins and associated limestone alteration in the Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A. Calcite from small, millimeter-wide veins has similar 87 Sr 86 Sr ratios (0.70781–0.70792) and δ 13C-values (−1.4 to +1.6‰, PDB) to host Viola limestone ( 87 Sr 86 Sr : 0.70779–0.70794; δ 13C: −2.2 to +1.7‰, PDB), indicating that the fluids from which the vein calcite precipitated were chemically buffered by the host limestone. The low δ 18O-values (−8.2 to −5.5‰, PDB) of the vein calcite, relative to the host limestone (−5.0 to −3.1‰, PDB), suggest that the vein calcite probably precipitated from 18O-depleted meteoric water. Calcite from large veins (5–50 cm wide) is characterized by radiogenic 87 Sr 86 Sr ratios (0.70887–0.70915) and low δ 13C-values (−8.0 to −2.5‰, PDB), relative to host Viola limestone. Such isotopic compositions demonstrate that the fluids responsible for the formation of the large veins were enriched in 87Sr and depleted in 13C, relative to the host limestone. The δ 18O-values (−4.8 to −3.7‰, PDB), of calcite from the large veins, coupled with the presence of hot (50–90°C) and saline fluid inclusions, constrain the formation of the vein calcite to 18O-enriched ( δ 18 O > +2‰ , SMOW) fluids. On this basis, calcite from the large veins is believed to have precipitated from basin-derived fluids. Adjacent to the large veins, alteration zones up to 0.5 m wide have developed in host limestone. Relative to other limestones, the altered limestones display radiogenic 87 Sr 86 Sr ratios (0.70792–0.70880), low δ 13C-values (−8.2 to −2.0‰, PDB), and low Sr concentrations. The similar isotopic compositions of the altered limestone to those of the large veins suggest that alteration of limestones was also caused by basinal fluids.