Formation of Replacement Dolomite in the Latemar Carbonate Buildup, Dolomites, Northern Italy: Part 1. Field Relations, Mineralogy, and Geochemistry

Replacement dolomite in the Latemar carbonate buildup, northern Italy, formed when limestone was infiltrated by and reacted with Mg-rich fluid. It occurs in discrete bodies in sharp contact with unreacted limestone. The dolomite developed in a nearly orthogonal lattice of vertical columns (replacement of limestone breccia pipes) and sheets (replacement along fractures and limestone-dike contacts) and of nearly horizontal bedding-parallel sheets and tubes. Mapped patterns of replacement dolomite directly image that part of the plumbing system in which the amount of fluid flow was sufficient to form dolomite. Decreases in the proportion of dolomite relative to limestone and in the proportion of vertical relative to horizontal dolomite-limestone contacts with increasing elevation indicate that the overall direction of fluid flow was upward and then outward along more permeable bedding horizons. Dolomite is significantly enriched in Fe, Mn, and Zn, as well as in Mg, relative to calcite in precursor limestone but not in Cu, Ni, Co, Cr, Ba, or Pb. The Fe, Mn, and Zn content of dolomite varies spatially within outcrops from the scale of meters down to the micron scale of oscillatory growth zoning in individual dolomite crystals. The variation is interpreted in terms of a dolomitizing fluid that, unlike unmodified seawater, contained significant amounts of Fe, Mn, and Zn, as well as of Mg, and whose composition varied in space at a range of scales and in time at the site of growth of individual dolomite crystals. A nearly complete overlap in the C of dolomite (2.0 4.6‰, VPDB) and calcite (1.1 4.0‰) is evidence that the C of most dolomite was inherited directly from the calcite precursor. Measured O of dolomite has a wide range (21.8 27.7‰, VSMOW) overlapping with that of calcite (23.4 28.5‰) but shifted to lower values. Dolomite with O <23.4 permil could not have been equilibrium with any analyzed calcite at any temperature. The ranges in ODol and OCal and values of ODol <23.4 permil both indicate that O of calcite and dolomite were set by oxygen isotope exchange with the same fluid over a range of temperatures, with isotopically different fluids, or both.