Anacortes holds a 100% interest in the Tres Cruces Mineral concessions through its wholly owned subsidiary, Aurífera Tres Cruces S.A. (ATC).
On June 28, 2023, Steppe Gold Ltd. completed the acquisition of all of the issued and outstanding common shares of Anacortes Mining Corp.
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Summary:
The Tres Cruces property hosts gold deposits that have characteristics of a low- to intermediate-sulphidation type epithermal system. Epithermal gold-silver deposits are shallowly formed vein, stockwork, disseminated, and replacement deposits that are mined primarily for their gold and silver contents; some deposits also contain substantial resources of lead, zinc, copper, and/or mercury. Epithermal gold-silver deposits range in size from 10,000 to >1 billion tonnes and have gold contents of 0.1 to >30.0 g/t, and silver contents of <1 to several thousand g/t. Although epithermal deposits are commonly known for their high gold grades, many bulk tonnage deposits with as little as 1 g/t gold or less are presently being exploited by open-pit mining.
Gold mineralization at Tres Cruces is mainly associated with pyrite, with occasional traces of other sulphide minerals. The auriferous pyrite is very fine grained, gray to blackish gray (commonly called "fine black pyrite”) and is present as disseminations and in very thin veinlets that locally form areas of stockworks (Macedo et al., 2012). Black pyrite also occurs as rims on earlier pyrite forming a colloform-banded texture. Clemson (Battle Mountain Canada Ltd., Tres Cruces Staff, 1999b) noted colloform-banding for this stage of pyrite, often with repetitive oscillatory bands of sooty and crystalline pyrite. The late-stage black pyrite is directly associated with the main-stage gold event. Early formed, non-gold-bearing pyrite is coarsely crystalline, anhedral to euhedral, poikilitic, and is evenly disseminated through the andesitic rocks. A second stage, also coarsely crystalline, is subhedral to euhedral and occurs primarily in veinlets and fractures. In addition, minor amounts of marcasite, arsenopyrite, galena, stibnite, realgar, orpiment and enargite have been identified in some mineralized core. Some carbonate minerals and rhodochrosite are also found in areas of pyrite veining. Age of mineralization is bracketed between measured age dates of 25.1 Ma from andesite porphyry host rocks and 22.2 Ma from a post-mineral flow; however, there may have been multiple pulses of mineralizing fluids uring this period.
Advanced Mineral Technology Laboratory Ltd. (AMTEL), in a document by Battle Mountain Canada Ltd. Tres Cruces Staff (1999a) also completed a study on gold deportment on several core samples. The main forms of gold were found as free native grains, grains in pyrite and in gangue. AMTEL reports the microcrystalline pyrite contains very high concentrations of gold and arsenic (up to 141 g/t and 10%, respectively). There is a strong positive correlation of the gold in pyrite with its arsenic content. AMTEL also commented that the high surface area of the fine-grained pyrite would make it more susceptible to oxidation.
Gold deposition appears to have been controlled by both structural and stratigraphic features. Subvertical controls of the mineralization are located along the intrusive contacts of andesite porphyry and dacite porphyry bodies. Steeplydipping, commonly northeast-trending faults that cut the intrusive bodies may have provided deep-seated conduits for mineralizing hydrothermal solutions driven by heat from underlying magma. The rising high-pressure fluids caused the formation of breccia “pipes” containing fragments of sandstone that were carried up from older underlying rocks. Hydrothermal fluids carrying mineralization rose through the breccia and emanated laterally into adjacent flows, flow breccias and pyroclastic rocks. Rhyolitic pyroclastics in the upper part of the pile may have been less permeable than the underlying andesitic rocks, thereby causing the mineral-rich hydrothermal fluids to pool under the rhyolite and spread laterally for considerable distances. Decreasing temperature and/or pressure of the hydrothermal fluids initiated the deposition of gold and sulphide minerals within stockwork veins in the fractured host rocks. Mineralization has been found to extend only short distances into andesitic and dacitic intrusive rocks, which may be due to lower permeability or less favorable chemical characteristics.
The Tres Cruces deposit contains both oxide and sulphide mineralization. Gold-bearing sulphide mineralization is mainly hosted in the older andesitic pyroclastic rocks and to a limited extent in andesite porphyry. The rhyolites also host mineralization but only near or along the andesite–rhyolite contact. Most of the rhyolite hosted mineralization along this contact has been oxidized, possibly due to strong fracturing of the siliceous rhyolite, which allowed deep circulation of surface waters. The underlying andesitic rocks have developed only weak oxidization of the sulphide minerals, believed to be due to the abundance of clay minerals resulting in poor permeability (Battle Mountain Canada Ltd., Tres Cruces Staff, 1999b).