GEOCHEMISTRY OF PRECAMBRIAN CARBONATES .7. BELT SUPERGROUP, MONTANA AND IDAHO, USA

Carbonates from the similar to 1100-1450 Ma old Proterozoic Belt Super group were collected from stratigraphic sections throughout Montana an d Idaho, USA. The sampled sequences, in ascending stratigraphic order, include the Newland, Altyn, Spokane/Greyson transition, Empire, Walla ce, Helena, Siyeh, Snowslip, Shepard, and Libby formations. An increas e in the degree of postdepositional alteration of Belt limestones is r eflected in a diminution of Sr and Mg contents, an increase in Mn, and depletion in C-13 and O-18. Two diagenetic trends can be resolved for the limestones. One, affecting the presumed originally aragonite-rich sediments, includes carbonates from the Lower Belt Newland Formation. In contrast, the Middle Belt Carbonate (Wallace, Helena, Siyeh format ions) may have been originally of high-Mg calcitic mineralogy. Project ion of the alteration trends for the Lower and Middle Belt limestones suggest similar to 21 parts per thousand SMOW and similar to +2.5 part s per thousand to +1.0 parts per thousand PDB as the best preserved va lues for the delta(18)O and delta(13)C of seawater, respectively; both comparable to results from other Mesoproterozoic carbonate sequences. The oxygen isotope data for limestones show a regional westward deple tion of similar to 8 parts per thousand in O-18, possibly reflecting a higher temperature of postdepositional alteration in the western Belt basin. This depletion in O-18 is accompanied by a comparable decrease in delta(13)C values, most likely because a higher proportion of carb on was incorporated from CO2 generated by thermal cracking of hydrocar bons at depth. Dolostones in the Belt basin are dominantly micritic, w ith good preservation of depositional textures. Chemically and isotopi cally, their alteration trends mimic those of limestones, leading to c omparable projected ''best'' values for delta(13)C. Sr-87/Sr-86 values of Belt carbonates range between 0.70484 and 0.74991. Progressive dia genesis, as indexed by decreasing concentrations of Sr and depletions in O-18 and C-13, results in an increase in Sr-87/Sr-86 values. The le ast radiogenic measurement, from a Lower Belt Newland limestone sample , appears to fit reasonably well into the general trend for Proterozoi c seawater, as discussed in Mirota and Veizer (1994). This and the con sistency of delta(18)O and delta(13)C in limestones with other coeval sequences suggests that the (Lower and Middle) Belt carbonate sections are marine, deposited in an environment that was not continuously sep arated from the open ocean.