Sulfur and lead isotope studies of stratiform Zn-Pb-Ag deposits, Anvil Range, Yukon; basinal brine exhalation and anoxic bottom-water mixing

Abstract

Five stratiform Zn-Pb-Ag deposits are known in Early Cambrian metapelitic rocks along a curvilinear trend in the Anvil Range, central Yukon. The Anvil Range deposits occur along the southwestern boundary of the Selwyn basin in the stratigraphic transition zone between metapelites of the Mt. Mye unit and calcareous phyllites of the overlying Vangorda unit. The massive sulfides are associated closely with anomalously thick graphitic phyllites, apparently related to a second-order basin. A typical Anvil cycle of mineralization begins with a ribbonbanded graphitic-quartzitic-pyritic unit. This grades upward into sulfide-bearing quartzite, quartzitic massive sulfide, massive sulfide, and finally a baritic massive sulfide horizon. Sericitic alteration envelopes irregularly encompass each deposit and locally are developed best in footwall rocks. Detailed sulfur isotope studies have been carried out on the DY and Grum deposits and on one representative drill hole from the Faro deposit. The b34S values of sulfide minerals generally range from 10 to 22 per mil and are similar in all three deposits. The b34S values of pyrite in unmineralized samples from the district exhibit a wider range, from 6 to 34 per mil, and show distinct upward stratigraphic increase due to a stagnation cycle in the basin. The ba4S values of barite samples are strongly dependent on bottom-water conditions and mode of mixing during brine exhalation. They range from 22 to 26 per mil in the Faro deposit to 36 to 42 per mil in the DY deposit. These variations are due to mixing of isotopically light sulfate (18-20%0) in Ba-bearing ore fluid and isotopically heavy residual sulfate (30-60%0) in anoxic seawater.Thirty-eight samples of galena from the DY, Grum, Faro, SB, and Swim deposits have been analyzed for lead isotope ratios. In general, the lead isotope ratio data indicate an upper crustal lead source, with the Proterozoic Grit unit which is inferred to underlie the district being the most likely source rock. A small component of mantle leads from mafic igneous rocks or due to source rock inhomogeneity is also indicated.Graphitic host lithologies, lack of stratiform iron oxides, ba4S values of sedimentary pyrite, and sulfide sulfur-organic carbon variations in unmineralized cores indicate formation of the Anvil deposits in strongly reduced bottom water related to a previously unknown Early Cambrian anoxic event.

Genesis of the Aguilar zinc-lead-silver deposit, Argentina; contact metasomatic vs. sedimentary exhalative

“…They are stratigraphically controlled, strikingly not be carbonate layers, as would be typical for skarn deposits, but rather by medium-to fine-grained siliciclastic rocks. The geometry of the orebodies conforms to the distribution of sulfide accumulations in first-order, second-order, and third-order sedimentary basins often recognized in sedimentary exhalative sulfide accumulations (Large, 1983;Shanks et al, 1987). The environment of deposition in the large Cambro-Ordovieian basin ranged from platform to shallow marine to deep marine westward and represents the eastern reaches of a deep-marine, first-order basin (Sureda et al, 1986;Martin, 1987;Sureda and Martin, 1990 fides may be more proximal than those at the Esperanza prospect, which consists of only one small ore lens, a point further elaborated below (Fig.…”

Sulfur Isotope Geochemistry of Sulfide Minerals

“…Despite the predictable relationship between the δ 34 S of seawater and hydrothermal sulfi des, Janecky and Shanks (1988) found that sulfur isotope disequilibrium best describes sulfi de and sulfate in seafl oor vent systems, and that the systematic relationship is established at depth in the reaction zone of the seafl oor hydrothermal system. Sulfur isotope studies in the Selwyn Basin (Yukon) by Goodfellow (1987) and Shanks et al (1987) suggested the signifi cance of H 2 S-bearing anoxic bottom waters in determining the isotopic composition of sedimentary-exhalative massive sulfi de deposits. Goodfellow and Jonasson (1984) investigated the sulfur isotope composition of sedimentary pyrite and barite within the Cambrian to Mississippian strata of the Selwyn basin.…”

“…They used the higher values within the Selwyn basin as evidence that the Selwyn basin had restricted access to the open ocean and that the bottom waters were anoxic and H 2 S-bearing. Shanks et al (1987) extended the Selwyn basin curve farther back into Cambrian time with additional data from the Anvil district. They used data from sedimentary pyrite to defi ne a secular H 2 S curve.…”

Contribution to the genesis of the SEDEX-type mineralizations of the Rhenish Massif (Germany) — implications for future Pb-Zn exploration

“…GUSTAFSON &? The Canadian Selwyn Basin was an incipient rift basin on Proterozoic (epicratonic) continental crust filled with clastic sediments during Late Proterozoic to late Paleozoic times (GooI) F~LLOW, 1986;SHANKS et al, 1987). The SEDEX deposits of Australia are situated within prominent intracontinental rifts of Middle to Upper Proterozoic age (DuNNET, 1976;WLLLIAMS, 1978).…”