Sulfur isotope evidence for penetration of MVT fluids into igneous basement rocks, southeast Missouri, USA

Abstract

Abstract. Previous studies of galena and sphalerite fromPaleozoic MVT deposits in the Viburnum Trend, southeast Missouri documented large variations in 634S values throughout the ore-forming event. The present study of Cu-Fe-sulfides reveals a similar 634S variation that reflects two end-member sulfur reservoirs whose relative importance varied both temporally and spatially. More 34S-enriched sulfides (634S approaching 25%0) indicate introduction of sulfur from basinal sedimentary sources, whereas more 32S-enriched sulfides (63~S < 5%0 ) may reflect fluids moving through underlying granitic basement. Two areas containing Precambrian, igneous-hosted FeCu mineralization in southeast Missouri (West and Central Domes of Boss-Bixby) were investigated to elucidate their relationship to Cu-rich MVT orebodies hosted nearby within the overlying Cambrian Bonneterre Dolomite. Mineralization at Boss-Bixby is composed of an early phase of iron oxide deposition followed by Cu-Fe-sulfides. The Central Dome is faulted and its mineralization is more fracture-controlled than the typically podiform ores of the West Dome. The 634S values of West Dome sulfides are 0.9 to 6.5%0 and pyrite-chalcopyrite indicate a temperature of 525~ 50~These data indicate an igneous source of sulfur during Precambrian ore deposition. In contrast, 634S values of Central Dome sulfides are 9.4 to 20.0%0 and pyrite-chalcopyrite indicate temperatures of 275 ~ + 50 ~ Similar 634S values are obtained for chalcopyrite from the overlying MVT deposits. We speculate that deeply circulating, basin-derived MVT fluids mobilized sulfur and copper from the underlying igneous basement and redeposited them in overlying Curich MVT orebodies, as well as overprinting earlier Precambrian sulfides of the Central Dome with a later, Paleozoic MVT sulfur isotope signature. Many models for MVT fluid circulation in the Midcontinent region of North America assume that igneous basement rocks are an impermeable boundary, but in southeast Missouri, evidence exists for structurally controlled MVT fluid movement > 600m vertically through underlying Precambrian igneous rocks. Such basement involvement has been suggested for other carbonate-hosted base-metal districts (e.g. Irish base metal deposits) and should be considered an integral part of the ore-forming process in southeast Missouri. Geological settingSoutheastern Missouri is part of the stable cratonic interior region of North America.

Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits

“…Gneissic deep basement rocks can attain 40 Ar/ 36 Ar values as high as 37,200 (Drescher et al, 1998). Recently, basement interaction has also been favored for Irish-type Pb-Zn deposits and some MVT districts (e.g., Shelton et al, 1995;Spirikis, 1995;Everett et al, 1999), and widespread basement-penetration by sedimentary brines is documented by Pb-Zn mineralized quartz veins in basement rocks elsewhere in the European continent (Gleeson and Yardley, 2002).…”

Replacement Dolomites and Ore Sulfides as Recorders of Multiple Fluids and Fluid Sources in the Southeast Missouri Mississippi Valley-Type District: Halogen-87Sr/86Sr- 18O- 34S Systematics in the Bonneterre Dolomite

“…Several potential sulfur reservoirs exist for southeast Missouri MVT ores, encompassing a wide range of δ 34 S values. Sulfides in local igneous rocks have a range of values from 0 to 7 per mil (Balogh et al, 1988;Shelton et al, 1995). Lower Paleozoic seawater and sedimentary sulfates have δ 34 S values of ~15 to 30 per mil (Claypool et al, 1980).…”

“…10) also reflects the presence of a second brine type, highly evaporated seawater, during the Early stage, which may imply that fluid mixing was an important ore-forming process. The δ 34 S values (<6‰) are interpreted to indicate that sulfur was derived locally from sulfides in sedimentary and igneous basement rocks (Shelton et al, 1995).…”

Tri-State and Viburnum Trend Districts: An Overview

“…A relatively new mineral, fletcherite, was first described from a bornite lense in the Fletcher mine (Craig and Carpenter 1977). Some of the largest djurleite crystals known (0.75 inch) were found in the Sweetwater mine (Hagni 1995). Currently, characterization is being conducted by Hagni on a copper-molybdenum mineral from a bornite pod that closely resembles the partially described mineral castaingite (R. Hagni 1996, pers.…”

“…These pods, consisting mostly of bornite and shown as a paragenetic diagram in figure 34, contain minute inclusions of copper-cobalt-nickel minerals such as cobalt-pyrite, bravoite, nickel-carrolite, gersdorffite, tennantite, chalcopyrite, enargite, and vaesite. Chalcocite veinlets cutting the bornite pods contain djurleite, anilite, digenite, covellite, and blaubleibender covellite (Shelton et al 1995). For additional information on blaubleibender covellite, see Goble (1978).…”