Approximate Age of Tectonic Activity Using Anomalous Lead Isotopes

Abstract

A postulate has been added to the Russell-Farquhar model for anomalous leads which yields a quantitative estimate of the time of their mineralization. Consider tl to t e a time of tectonic activity during which ordinary leads were differentiated from a deep source and either concentrated to form a lead deposit at this time or disseminated throughout the upper crust. Contemporaneously uranium and thorium bearing solutions were incorporated into the same environment. At time t2 tectonic activity recurred in the area. Some of the ordinary leads were remobilized and became contaminated with radiogenic lead to form anomalous lead deposits. Existing lead isotope analyses yield the following results. At Broken Hill, Australia, tl is I 600 million years while t2 is 510 & 80 million years. For Goldfields, Saskatchewan, tl is 2015 My while t2 is 5605 250 My. At Sudbury, Ontario, tl is 1730 My, and t2 is 870 My. For lead deposits around the Ozark Dome area, including Joplin and Bonne Terre, Missouri, tl is 1350 My and tz is about 115 My. For leads in west-central New Mexico, tl is I490 My and t2 is about 69 My. The errors in the estimates are generally quite large but the values quoted are consistent with available geologic data.

The Application of Lead Isotopes to the Problems of Ore Genesis and Ore Prospect Evaluation: A Review

“…If the age of mineralization is known approximately, the age (perhaps integrated ages) of the source material for the lead may be calculated, as was done by Stacey et al (1967). Some other investigators have tried to apply the secondary isochron in the reverse manner; that is, calculate the mineralization age after an estimate has been made for the source material of the lead (Kanasewich, 1962). Using these tables, an actual mineralization age of 50 m.y., rather than zero, would only reduce the calculated age of the source material by about half that amount, 25 m.y.…”

“…Because the equation for secondary isochrons is transcendental, the solution is one of trial and error, but the tables of Stieff et al (1959) or the revised tables by Stacey and Stern (1973) may be used if the mineralization age is near zero. The secondary isochron method may sometimes find application in estimating mineralization ages where a lack of precision is not the key issue, as was done by Kanasewich (1962) in estimating that the age of mineralization of the Thackaringa-type deposits in the Broken Hill area of Australia was about 500 m.y. Some other investigators have tried to apply the secondary isochron in the reverse manner; that is, calculate the mineralization age after an estimate has been made for the source material of the lead (Kanasewich, 1962).…”

“…For other extended reviews of the various aspects of lead isotopes covered in this paper, the reader is referred to Russell and Farquhar (1960), Kanasewich (1968), Stacey et al (1968), Doe (1970), Russell (1972), and Gale and Mussett (1973). We will attempt to show here that lead isotope investigations deserve to be included in the complement of tools used in the difficult job of ore prospect evaluation.…”

Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs

“…There are several ways of generating linear regression lines on Pb isotopic ratio plots: closed system multi-stage evolution with random variation of /^(238U / 204Pb) values; continuous in crease of imposed on the multi-stage evolution; two-stage episodic mantle differentiation (see, for example, Kanasewich 1962;Cooper & Richards 1966;Oversby & Gast 1970); mixing of two mantle components (see, for example, ; continuous mantle-crust mixing (Armstrong 1968;Armstrong & Hein 1973); magma chamber contamination (Oversby & Gast 1970;Tatsumoto 1978).…”

Lead isotopes from the Upper Mississippi Valley district: A regional perspective

“…Heyl and others (1966), Yonk (1970), and Richards and others (1972) used ^=1,350 Ma, an average age of basement rocks in the Upper Mississippi Valley as determined by Kanasewich (1962), Tilton and others (1962), Goldich and others (1966), Muehlberger and others (1966), and Lidiak and others (1966). Subsequent studies indicate that this choice of t\ might be too young.…”