Correspondence of ores of silver and gold with basement terranes in the American southwest

Abstract

Abstract. The ratios of silver to gold produced from epigenetic ore districts of the American southwest reveal a consistency of value ranges, differing by an order of magnitude, that may be identified with either one or the other of two geologic terranes in which the ores occur. A discriminating value of the ratio is about 17.5 : 1, the ratio of crustal abundance given by Ahrens (1965). (No further significance is attributed to this value, at this time, beyond the fact that it appears to establish a reasonable separation of values on the basis of geographic occurrence.) Ores relatively enriched in Ag occur in terranes floored by thick Proterozoic clastic and Paleozoic marine successions, and ores relatively enriched in Au lie above or within a Proterozoic basement dominated by maficfelsic volcanic (arc) successions. Proterozoic granites occur in each region. The values of the ratio are broadly consistent within each terrane, irrespective of the age of ore formation, the ore deposit style, associated igneous rocks, structural control, differing interpreted styles of subduction, and weathering histories. These characteristics and associations support a hypothesis that metallogenic signatures of ore districts in this region are fundamentally related to the crust in which the ores occur.Regional analysis of metals in Arizona ore deposits has led to a hypothesis that the silver and gold of districts may be inherited from the crust (Titley 1987(Titley , 1989. Limiting values of ratios of Ag:Au produced from ores of districts correspond geographically to the distribution of different ages and compositions of subjacent Proterozoic basement. The correspondence with basement compositions transcends the genetic style of ores, the tectonic environment and setting of ore districts, apparent associated igneous rock types and the ages of metallogenesis. Further, the orders of magnitude of the values of the ratio correspond in a general way to those values inferred from published data for known basement lithologies.The original regional analysis (Titley /987) was restricted to the State of Arizona because of limited data. This report extends that metallogenic data in New Mexico from information recently published by North and McLemore (1988) and into parts of Nevada and California from some data of Wilkins (1984). Boundaries of Proterozoic basement lithologies and corresponding ages in Arizona have been extended into adjoining New Mexico. The results of this expansion of basement and precious metal characteristics show that the correspondence of the Ag:Au ratios with distinctive Proterozoic basement of the American Southwest appears to be regionwide. Values of the Ag : Au ratio from districts within or above the older, volcanic-dominated Proterozoic basement are relatively enriched in gold (< 17.5), those above the younger Proterozoic, clastic rock-dominated basement, relatively enriched in Ag.

Jurassic to Holocene tectonics, magmatism, and metallogeny of northwestern Mexico

“…The provinciality of metal assemblages (Fig. 11) has been interpreted as reflecting differences in crustal composition (e.g., Titley, 1991), but could also be influenced by differences in host rocks, available fluids, or shifting locus of magmatism. The eastward increase in Pb and Ag relative to Cu and Au thus may reflect the increasingly felsic, Pbrich, Cu-poor crust with the transition from crust formed in the western-domain continental volcanic margin to crust formed in the eastern-domain Precambrian basement and platform carbonates (Fig.…”

About metallogeny of the pacific volcanic belts

“…Выполненный пространственно-статистический анализ имеющейся базы данных показал, что месторождения краевых ТВП отличаются более высокими содержаниями Ag и, следова-тельно, характеризуются более высокими Ag/Au отношениями, чем месторождения островных вулканических дуг . Полученные значения Ag/Au по месторождениям ТВП подтверждают положение [Titley, 1991] Таким образом, истинная зональность оруденения в ТВП определяется главным образом тем, что колчеданные и медно-порфировые ряды рудных формаций развиты преимущественно в пределах островодужных террейнов и внутренних (по отношению к океану) частях окраинно-континентальных вулканогенных поясов, которые обычно развиваются на островодужных террейнах . Однако в ряде случаев появляются также убедительные сведения о металлогенической унаследованности кайнозойского островодужного оруденения (Au-Ag-Sn-W) от докембрийского фундамента [Milesi et al ., 1994] .…”

Source and redox controls on metallogenic variations in intrusion-related ore systems, Tombstone-Tungsten Belt, Yukon Territory, Canada

“…The tectonic setting, magma composition, degree of fractional crystallisation and redox state all potentially play a role in determining the metallogeny of an intrusion-related mineral deposit district (e.g. Ishihara 1981;Titley 1982Titley , 1991Blevin & Chappell 1992;Barton 1996;Blevin et al 1996;Lang & Titley 1998). Controversies have typically focused on the relative importance of either the source materials or magmatichydrothermal processes in enriching a system in a particular metal suite.…”