Print this pageNELSON, CARL E., Consultant, Boulder, CO
ABSTRACT: Comparative Geochemistry of Jasperoids from Carlin-Type Gold Deposits of the Western United States
Exploration for Carlin-type gold orebodies in the western United States typically involves sampling and analyzing jasperoid, a distinctive alteration type formed by intense silicification of marine sediments. In this study, rock suites were collected from six orebodies and four similar, but barren, systems. Jasperoids at all ten systems contain episodically silicified breccias; quartz vein stockworks; elevated AS, Sb, Hg, Ba, and Tl; and, locally, anomalous Au and Ag. Jasperoids from the four barren systems are as anomalous as jasperoids from the six orebodies in all members of an epithermal geochemical suite, including gold and silver.
A database suite containing 272 samples from six of the ten systems was analyzed for 45 elements. Q-mode factor analysis shows that geochemical variance in the epithermal geochemical suite is related to geochemical variance in Li, P, Mn, Ba, Mo, Cr, Co, V, Cd, Ni, U, Zn, and Pb. Metalliferous marine black shales are enriched in these elements and are spatially related to Carlin-type deposits.
A test suite containing 109 samples from the four remaining systems was analyzed to determine whether a discriminant function derived from the database suite could be applied to exploration. An eight-element function correctly assigned 40 of 42 jasperoid samples collected from the surface at the Carlin and Horse Canyon gold deposits. These results indicate that elements characteristic of metalliferous marine black shales can be used to identify Carlin-type systems with associated gold ore. Gold in Carlin-type orebodies may have been leached from a source-rock sequence containing metalliferous marine black shales.
AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.