Scapolite in the Belt Series in the St. Joe-Clearwater Region, Idaho

Abstract

SCAPOLITE is a common rock-forming mineral in parts of the Belt Series of Precambrian age in southern Shoshone County and adjoining parts of Clearwater County, Idaho. It is most abundant in moderately metamorphosed calcareous shaly layers of the Wallace Formation but occurs also in their highly metamorphosed equivalents and in the lowest part of the Prichard Formation. The mode of occurrence varies with distance from the Idaho batholith, with bulk composition, and with grade of metamorphism. In the northern part of the area where rocks were metamorphosed to the epidote-amphibolite facies, the highest concentration of scapolite is in layers rich in calcite, biotite, or hornblende and diopside. In biotite-rich layers interbedded with quartzite, scapolite is in round holoblasts; in carbonate granofels, crystals are euhedral to subhedral; and in hornblende-and diopside-bearing layers, small anhedral grains are common. In rocks metamorphosed to the amphibolite facies, such as diopside gneiss and calcium-magnesium-aluminum silicate rocks (here named "camalsite"), scapolite occurs in small anhedral grains. In diopside gneiss, scapolite is in thin layers that extend long distances parallel to the bedding. In camalsite, small masses exceptionally rich in scapolite are common. The mode of occurrence and the distribution parallel to the bedding suggest that scapolite crystallized from a sedimentary rock that contained saline minerals. The scapolite contains much CI but only a little S03, suggesting that halite was the chief source mineral. The local high concentrations of scapolite occur in a zone where elements have been redistributed, either because of metamorphism of dispersed or layered saline minerals with accompanying migration of chlorine or because of metamorphism of primary local masses of such minerals.

Iron and chlorine as guides to stratiform Cu-Co-Au deposits, Idaho Cobalt Belt, USA

“…Cl-rich biotite and scapolite in the Belt Supergroup of northern Idaho (Heitanen 1967;Mora and Valley 1989), are very similar to the phases in the Yellowjacket and would seem to be petrogenetic analogs indicative of formation from halite-bearing beds. However, there are important geologic differences: unlike the Belt rocks that have abundant evidence for shallow-water sedimentation (e.g., Grotzinger 1986), features in the lower and middle Yellowjacket suggest deep-water sedimentation , and our studies could find no red strata, desiccation cracks, or other evidence for evaporite conditions.…”

“…One possible source of C1 is evaporites; however, the sedimentology and bulk composition of these Cl-rich rocks differ significantly from occurrences interpreted as meta-evaporites (e.g., Heitanen 1967;Serdyuchenko 1975). Trace elements, ratios such as CI/Br, and stable isotopic data have been cited as evidence for evaporites, but the many stages of superimposed alteration and metamorphism in the Idaho Cobalt Belt probably would change original concentrations, thus data should be interpreted with caution (Hardie 1984).…”

“…Halite is known in the Belt Supergroup in northern Idaho and Montana (Heitanen 1967;Mora and Valley 1989), but evidence for it is lacking in the Yellowjacket Formation. Only a few studies of diagenetic or low-grade metamorphic minerals report substantial amounts of C1.…”

Anomalous chlorine in iron-rich strata; Yellowjacket formation, Lemhi County, Idaho: Analytical data and discussion

“…We have seen no textures suggestive of evaporite minerals (such as halite casts), as are known in the Wallace and Missoula Formations of the Belt Supergroup in northern Idaho and Montana (Heitanen, 1967;Grotzinger, 1986). We have found no references to diagenetic or very low grade metamorphic silicate minerals that contain substantial amounts of Cl.…”

“…However, the stratigraphy and bulk composition of these Cl-rich rocks differ significantly from many classic scapolitic occurrences that have been interpreted to be meta-evaporites (Heitanen, 1967;Serdyuchenko, 1975;Ramsay and Davidson, 1970). The Cl-rich Yellowjacket strata are rich in Fe rather than Ca-Mg, and there are no associated carbonate strata or carbonate mineral cements.…”