--> Abstract: Diagenesis of the Lower Ordovician Ogdensburg Formation, Northern New York and Southeastern Ontario: Evidence for a Hydrothermal "Hot Spot", by K. J. Counter; #90987 (1993).

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COUNTER, KATHLEEN J., State University of New York at Binghamton, Binghamton, NY

ABSTRACT: Diagenesis of the Lower Ordovician Ogdensburg Formation, Northern New York and Southeastern Ontario: Evidence for a Hydrothermal "Hot Spot"

The Lower Ordovician Ogdensburg Formation (Beekmantown Group) of northern New York and southeastern Ontario has undergone at least two major diagenetic events: (1) massive replacement dolomitization and (2) Mississippi Valley-type mineralization.

Field and petrographic studies show that the original peritidal carbonates of the Ogdensburg Formation have been almost completely replaced with mosaic dolomite. Preserved sedimentary structures demonstrate that dolomite crystal size mimics the grain size of the precursor carbonate phase. Late-stage minerals (quartz, calcite, saddle dolomite, pyrite, sphalerite, bornite, chalcocite, and barite) occur in vugs, veinlets, and breccia-fills. These MVT mineral assemblages are commonly stratabound, particularly instromatolite layers, where dissolution, brecciation, and mineralization have followed porosity trends in the host rock.

Fluid inclusions indicate that late-stage quartz and calcite formed from hot, Na-Ca-Cl-rich brines. Homogenization temperatures in fluid inclusionsrange from 180-320 degrees C in single crystals. First melting of ice in frozen inclusions occurs between -70 - -60 degrees C, final melt temperatures range from-37 - -30 degrees C, and halite daughter crystals disappear between +2.5 - +4.7 degrees C.This data has been used to calculate fluid inclusion composition using the Spencer-Moller-Weare (1990) specific ion-interaction model. Results are m<Na> = 3.0-3.8, m<Ca> = 1.5-2.25, and m<Cl> = 6.8-7.5. DCP-AES analysis of fluid inclusion leachates confirmed that Na and Ca are the major cations in fluid inclusion brines, with negligible amounts of K and Mg. Average cation mola ratios are Na:Ca:K:Mg = 10:6:0.88:0.6. Fluid inclusion studies suggest a hydrothermal "hot spot" in northern New York and southeastern Ontario. There, late-stage minerals formed from hypersaline brines at temperatures and salinities greater than those of other MVT deposits in the Appalachians.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.