A REVISED EMPLACEMENT HISTORY OF JURASSIC AND CRETACEOUS INTRUSIVES AND ASSOCIATED CU-FE-S MINERALIZATION WITHIN THE PLUMAS COPPER BELT OF NORTHEASTERN CALIFORNIA

The Plumas copper belt of the northeastern Sierra Nevada is a north striking belt of Cu-Fe-S mineralization associated with intrusives ranging from gabbro to granite including: the Engels (ES), Lights Creek (LCS), Genesee-Reward (GRS) and China Gulch (CGS) stocks, the Lucky-S quartz-monzonite (LS), and the Little Grizzly Creek granodiorite (LGC). Originally considered to be Early Cretaceous based on unpublished ca. 110 Ma K-Ar ages obtained in the early 1980's; our new data demonstrate these intrusives fall into three Mesozoic age categories: late Pliensbachian, mid-Tithonian, and late Aptian.

The hydrothermally altered LCS, host to the Superior mine, previously yielded SHRIMP U-Pb ages of ca. 184 and 178 Ma from two very small-n samples (Dilles and Stephens, 2011) the former obtained near the Superior ore body. The much larger-n samples we obtained from the LCS, including one from near the Moonlight ore body, have yielded near identical weighted mean LA-ICPMS U-Pb ages of 185.46 +/- 0.21, 185.03 +/- 0.64 and 184.8 +/- 0.85 Ma which confirm the older of the two Dilles and Stephens SHRIMP age results. While more than two dozen Cu-Fe-S prospects have been developed within the slightly alkalic, proximal-facies volcanic rock of the Middle Jurassic portion of the Mount Jura sequence (Christe, 2011), similar age stocks have yet to be identified within the copper belt. Instead, several Late Jurassic hypabyssal bodies, larger intrusives and granite porphyry dikes cut the southern LCS and Late Jurassic pyroclastic rock. Our new sample from the LS quartz-monzonite, host to the Lucky-S mine has yielded a weighted mean LA-ICPMS U-Pb age of 149.0 +/- 0.25 Ma which is comparable to a SHRIMP U-Pb age of ca. 148 Ma previously obtained from the adjacent CGS (Dilles and Stephens, 2011) but slightly older than a ca. 146 Ma (Ar-Ar plateau) age obtained from granite porphyry dikes associated with a heavily mineralized, unnamed stock in the upper reaches of Moonlight Creek (Dilles and Stephens, 2011). To date, only the GRS, associated with the Genesee-Reward mine, has been found to yield a Cretaceous LA-ICPMS U-Pb age (ca. 117 Ma; Christe and Pecha, 2012). The remaining important intrusives within the Plumas copper belt, including the LGC granodiorite, associated with the Walker Mine, and the ES, which hosts the Engels mine, remain subject of ongoing geochronologic study.