Manganese Resources Of The Olympic Peninsula Washington PDF Download

"The manganese deposits of the Olympic Peninsula occur in the lower part of the Lower to Middle Eocene Crescent Formation, a thick sequence of interbedded pillow basalts, massive basalts flows, and pelagic sediments deposited, for the most part, in a submarine environment. The Steel Creek deposit lies in the southeast portion of the Olympic Peninsula near Lake Cushman. The deposit is composed predominantly of bementite. Other minerals include braunite, quartz, hausmannite, jacobsite, hematite, rhodochrosite, manganoan calcite, calcite, pyrite, chalcopyrite, barite, azurite, and alachite. Textural examination of ore reveals several episodes of hypogene mineral formation. Braunite, hausmannite, jacobsite, quartz, and hematite are the first formed minerals. Later bementite and rhodochrosite formed essentially contemporaneously. This was followed by calcite, manganoan calcite, pyrite, chalcopyrite, and barite, which comprise less than five percent of the deposit. Crosscutting features indicate that there were at least two episodes of hematite, bementite, and quartz formation. Supergene minerals include amorphous black manganese oxides, azurite, and malachite. The Steel Creek deposit is proposed to have formed on the sea floor off the western coast of North America. Collision between the eastward-moving oceanic Farallon Plate and the North American continent obducted much of the basalt and overlying sediments onto the continental margin, resulting in their present setting on the Olympic Peninsula. Constraints on the conditions of formation are difficult due to the lack of thermodynamic data for bementite, and the multitude of variables. Broad parameters are proposed, however. This deposit formed by the process of hydrothermal solutions leaching Mn, Fe, Si, and other elements from the basalt and depositing them upon mixing with seawater. Temperatures probably did not exceed 200 C. The pH is estimated to have been between 6.0 and 8.0. Oxidizing conditions must have existed for the initlal deposition of the oxide minerals. The lack of abundant sulfide and sulfate minerals indicates that sulfur was a minor component. However, a sulfide-rich zone may have been eroded away or deposited distal to this deposit. Several questions concerning bementite were raised during the course of this study. Conflicting x-ray evidence suggests that the "bementite" at Steel Creek may not be the same as that from the type locality at Franklin, New Jersey. This may also be true of the bementite from other Olympic Peninsula manganese deposits"--Document.