Abstract

The Lights Creek stock is located in the northern Sierra Nevada of California and is intruded into Mesozoic metavolcanic rocks with lesser gabbro and diorite. The stock has a surface area of about seven square miles and consists mainly of quartz monzonite and granodiorite with lesser granite and subvolcanic rock equivalents. Porphyry copper-type mineralization is present in several broad marginal areas.Rock samples were collected from 98 cores and 31 surface outcrops distributed over the stock with an elevation range of 2,700 ft, and analyzed for copper, lead, and zinc. In spite of large variance (coefficient of variation up to 76%) at the outcrop or local scale, significant regional distributions are also present which, in the case of copper, correlate with the areas of mineralization as well as with some variations in the localscale variance.Analysis of frequency distributions of pooled local-scale metal data suggests the copper mineralization has been introduced on a scale larger than that of individual drill cores, and is not merely a small-scale segregation of magmatic-stage constituents into subsequent adjacent fractures.In three-dimensional regressions for the three elements, the only significant trend in the vertical dimension occurs in the regression on copper, with a pattern of marginal or peripheral "peel-like" concentrations which increase with elevation. These areas of variable mineralization tend to surround a barren or low copper (< 200 ppm) core in the north-central portion of the stock.Although the base metal regression patterns and frequency distributions do not superficially serve to select among processes proposed for porphyry copper mineralization (i.e., magmatic-hydrothermal and convecting meteoric or formation waters), their relationship to certain geologic features suggests a largely magmatic source of the metals in this stock. Among these features are the locations of some orebodies, similarities of mineralization within and outside the stock, contrasts in metal contents of xenoliths and wall rocks or inliers at the contacts and within the stock, and the presence of variable disseminated sulfides in all rock facies of the stock.

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