ABSTRACT: Precipitation of Saddle Dolomite Cement by CO₂ Effervescence from Upward-Migrating Basinal Brines

D. L. Leach, G. S. Plumlee, A. H. Hofstra, G. P. Landis, E. L. Rowan, J. B. Viets

Saddle dolomite, a common gangue mineral in Mississippi Valley-type (MVT) ore deposits of the Ozark region, occurs throughout much of the Paleozoic stratigraphy of northern Arkansas, southern Missouri, and Kansas. Outside the main ore districts, saddle dolomite cement is found with trace amounts of sulfides in fractures, solution collapse features, a variety of vuggy and moldic porosity, and as intergranular cement.

Studies of fluid inclusions, cathodoluminescent microstratigraphy, and mineral parageneses, show that this regional saddle dolomite formed within the same hydrologic system, and from the same fluids ultimately responsible for forming the MVT districts. Mass spectrometry of the fluid inclusion gases shows the presence of both CO₂-rich and H₂O-rich inclusions in saddle dolomite and associated sulfides. These data indicate that CO₂ effervescence (boiling) from the mineralizing brine was a widespread phenomenon accompanying dolomite precipitation in ore zones and regionally. Gradual pressure decrease along an updip fluid flow path could account for the CO₂ effervescence.

Reaction path modeling of dolomite precipitation, using quantitative fluid compositions determined by extraction of fluid inclusions, indicates that near-isothermal separation of a CO₂-rich gas phase through boiling or effervescence from a brine of about 5 molal total dissolved solids will precipitate saddle dolomite cement and minor sulfide. However, other mechanisms, such as cooling or fluid mixing, must be invoked to explain the deposition of ore-grade sulfides. In the Ozark region, reaction path calculations indicate that a wide range of fluid compositions, under hydrostatic pressure, should have boiled at depths up to 2 km. Effervescence of CO₂ from brines gradually migrating to shallower stratigraphic levels may explain the occurrence of saddle dolomite cem nt in many sedimentary basins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990