An Investigation of Fluid Inclusions and Geochemistry of Ore Formation in the Cedar Creek Breccia Pipe, North Santiam Mining…

The Cedar Creek breccia pipe is located in the North Santiam Mining District, Oregon, approximately 50 km. east of Salem. It was emplaced in the Sardine Formation, a series of andesitic flows, breccias, tuffs, and small intrusives of Middle to Late Miocene age. The breccia pipe was discovered by Amoco Minerals Co. through soil sampling, a ground magnetic survey, and drilling. It is elliptical in plan view with maximum axes of approximately 110 and 145 m. and extends vertically downward over 350 m. The contacts between the pipe and the surrounding country rock are sharp and characterized by sheeted fault zones. A quartz diorite intrusive is located directly beneath the pipe. It is porphyritic, weakly mineralized, and exhibits quartz-sericite alteration. Two overlapping stages of hydrothermal mineralization are recognized. The first involved the alteration of breccia fragments to fine-grain quartz, sericite, chlorite, and carbonates. The second stage resulted in the deposition of quartz, sericite, chlorite, tourmaline, apatite, hematite, chalcopyrite, bornite, molybdenite, tetrahedrite, pyrite, galena, and sphalerite in open spaces. A later stage during which minor carbonate veins formed is also recognized. Siderite and koalinite were deposited during this late stage. Four types of fluid inclusions were distinguished and subjected to microthermometric analysis. Type I inclusions consist of vapor and liquid, and homogenized to liquid at temperatures of 150 degrees to 500 degrees C. The salinities of type I inclusions range from 0 to 23 equiv. wt. % NaCl. Type II inclusions consist of vapor, liquid, and halite. Homogenization temperatures of type II inclusions range from 200 degrees to greater than 625 degrees C. 65 type II inclusions homogenized to liquid by vapor disappearance and have salinities of 31 to 39 equiv. wt. % NaCI. 3 homogenized to liquid by halite disappearance and have salinities of 58 to greater than 65 equiv. wt. % NaCl. Type III inclusions consist of vapor, liquid, halite, and sylvite. 24 of 31 type III inclusions analyzed homogenized to liquid by vapor disappearance, the remainder by halite disappearance. Homogenization temperatures of type III inclusions range from 325 degrees to greater than 625 degrees C and salinities from 45 to 80 equiv. wt. % NaCl + KCl. Type IV inclusions consist of vapor, liquid, halite, sylvite, and one or more other solid phases. 2 of these solid phases were tentatively identified as gypsum and anhydrite. Homogenization temperatures of type IV inclusions range from 375 degrees to 525 degrees C. 32 homogenized to liquid by vapor disappearance, 13 by halite disappearance. Petrologic and fluid inclusion data indicate that 3 major hydrothermal events occurred. The first event is represented by type III and IV inclusions, the second by type II inclusions, and the last by type I inclusions. Fluid inclusion leachates were analyzed by ion chromatograph. Na, K, Li, NH4, Cl, SO4, NO3, and Br were separated. The Na/K and Na/Li ratios determined from the leachate analyses were used to calibrate Na/K and Na/Li geothermometers. Data from fluid inclusion and petrologic studies were combined with available thermodynamic data to create a model for ore transportation and deposition processes. Copper and iron were carried in solution as CuCl and FeCl+. Ore deposition occured at 250 degrees to 350 degrees C. A decrease in temperature is probably the primary cause of ore deposition, but an increase in pH and decrease in chloride concentration may also be important.