J. S. Cline
C.M. Fanning
J. L. Wooden
R. M. Tosdal
2003
<p>Lead<span> isotope compositions of bulk mineral samples (fluorite, orpiment, and realgar) determined using conventional techniques and of ore-stage arsenian pyrite using the Sensitive High Resolution Ion-Microprobe (SHRIMP) </span>in<span> the </span>Getchell<span> and </span>Turquoise<span> </span>Ridge<span> </span>Carlin<span>-</span>type<span> </span>gold<span> </span>deposits<span> (Osgood Mountains) require contribution from two different Pb </span>sources<span>. One Pb </span>source<span> dominates the ore stage. It has a limited Pb isotope range characterized by </span><sup>208</sup><span>Pb/ </span><sup>206</sup><span>Pb values of 2.000 to 2.005 and </span><sup>207</sup><span>Pb/ </span><sup>206</sup><span>Pb values of 0.8031 to 0.8075, as recorded by 10-μm-diameter spot SHRIMP analyses of ore-stage arsenian pyrite. These values approximately correspond to </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb of 19.3 to 19.6, </span><sup>207</sup><span>Pb/ </span><sup>204</sup><span>Pb of 15.65 to 15.75, and </span><sup>208</sup><span>Pb/ </span><sup>204</sup><span>Pb of 39.2 to 39.5. This Pb </span>source<span> is isotopically similar to that </span>in<span> average Neoproterozoic and Cambrian elastic rocks but not to any </span>potential<span> magmatic </span>sources<span>. Whether those clastic rocks provided Pb to the ore </span>fluid<span> cannot be unequivocally proven because their Pb isotope compositions over the same range as </span>in<span> ore-stage arsenian pyrite are similar to those of Ordovician to Devonian siliciclastic and calcareous rocks. The Pb </span>source<span> </span>in<span> the calcareous rocks most likely is largely detrital minerals, since that detritus was derived from the same </span>sources<span> as the detritus </span>in<span> the Neoproterozoic and Cambrian clastic rocks. The second Pb </span>source<span> is characterized by a large range of </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb values (18-34) with a limited range of </span><sup>208</sup><span>Pb/ </span><sup>204</sup><span>Pb values (38.1-39.5), indicating low but variable Th/U and high and variable U/Pb values. The second Pb </span>source<span> dominates late and postore-stage minerals but is also found </span>in<span> preore sulfide minerals. These Pb isotope characteristics typify Ordovician to Devonian siliciclastic and calcareous rocks around the </span>Carlin<span> trend </span>in<span> northeast </span>Nevada<span>. Petrologically similar rocks host the </span>Getchell<span> and </span>Turquoise<span> </span>Ridge<span> </span>deposits<span>. </span>Lead<span> from the second </span>source<span> was either contributed from the host </span>sedimentary<span> </span>rock<span> sequences or brought into the hydrothermal system by oxidized ground water as the system collapsed. Late ore- and postore-stage sulfide minerals (pyrite, orpiment, and stibnite) from the Betze-Post and Meikle </span>deposits<span> </span>in<span> the </span>Carlin<span> trend and from the Jerritt Canyon mining district have Pb isotope characteristics similar to those determined </span>in<span> </span>Getchell<span> and </span>Turquoise<span> </span>Ridge<span>. This observation suggests that the Pb isotope compositions of their ore fluids may be similar to those at </span>Getchell<span> and </span>Turquoise<span> </span>Ridge<span>. Two models can explain the Pb isotope compositions of the ore-stage arsenian pyrite versus the late ore or postore sulfide minerals. </span>In<span> either model, Pb from the Ordovician to Devonian siliciclastic and calcareous </span>rock<span> </span>source<span> enters the hydrothermal system late </span>in<span> the ore stage but not to any extent during the main stage of ore deposition. </span>In<span> one model, ore-stage Pb was derived from a </span>source<span> with Pb isotope compositions similar to those of the Neoproterozoic and Cambrian clastic sequence, transported as part of the ore </span>fluid<span> and then deposited </span>in<span> the ore-stage arsenian pyrite and fluorite. The second model is based on the observation that the Pb isotope characteristics of the ore-stage minerals also are found </span>in<span> some Ordovician to Devonian calcareous and siliciclastic rocks. Hence, ore-stage Pb could have been derived locally and simply concentrated during the ore stage. Critical to the second model is the removal of all high </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb (>20) material during alteration. It Also requires the retention of only the low </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb component of the Ordovician to Devonian </span>sedimentary<span> rocks. This critical step is possible only if the high </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb values are contained </span>in<span> readily dissolvable mineral phases, whereas the low </span><sup>206</sup><span>Pb/ </span><sup>204</sup><span>Pb values are found only </span>in<span> refractory minerals that released Pb during a final alteration stage just prior deposition of auriferous arsenian pyrite. Distinguishing between Pb transported with the ore </span>fluid<span> or inherited from the site of mineral deposition is not straightforward; however, it is simpler to explain the Pb isotope compositions of ore-stage arsenian pyrite and fluorite </span>in<span> two different but spatially related </span>Carlin<span>-</span>type<span> </span>deposits<span> (</span>Getchell<span> and </span>Turquoise<span> </span>Ridge<span>) with different host rocks by input of Pb with the ore </span>fluid<span>. Once the limited Pb </span>in<span> the hydrothermal </span>fluid<span> was exhausted by incorporation </span>in<span> ore-stage arsenian pyrite or other ore-stage minerals, Pb from the second </span>source<span>, the Ordovician to Devonian </span>sedimentary<span> </span>rock<span> sequences, became available for incorporation </span>in<span> some but not all of the late-stage sulfide minerals.</span></p>
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10.2113/gsecongeo.98.6.1189
en
Society of Economic Geologists
Lead in the Getchell-Turquoise ridge Carlin-type gold deposits from the perspective of potential igneous and sedimentary rock sources in Northern Nevada: Implications for fluid and metal sources
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