Roisin Kyne HBSc thesis abstract

Thesis Title: 
Clay Alteration of the Cerro la Mina Porphyry-Epithermal Deposit, Chiapas, Mexico
Roisin E.

The Cerro la Mina Cu-Au-Mo porphyry-epithermal prospect is located in southern Mexico in the state of Chiapas. Discovered in 2004 as part of a larger project, referred to as the Ixhuatán Project, Cerro la Mina is one of five deposits owned and operated by joint venture partners Linear Gold Corp and Kinross Ltd. The state of Chiapas lies within a tectonically complex region marked by the triple junction of the North American, Caribbean, and Cocos plates.  It is comprised of a pre-Mesozoic metamorphic and sedimentary basement, a sequence of Mesozoic sedimentary rocks, and a sequence of volcanic and sedimentary Cenozoic rocks. Cerro la Mina is situated within a volcanic belt known as the Chiapanecan Volcanic Arc (CVA) in an area of major faulting and fracturing. Host rocks at Cerro la Mina are comprised of sequences of andesitic volcanic rocks ranging from tuff-lapilli-breccia-size fragments and coarser dacitic pyroclastic units including flows, lahars, and debris flows. The majority of these host rocks have been highly altered to the point where advanced argillic alteration has obscured any primary textures. Zircon in the host rock at Cerro la Mina has been dated at 1.01± 0.04 Ma which is younger than the surrounding rocks which generally range in age from 2.14±0.04 to 2.79±0.08 Ma.

While much work has been done on Cerro la Mina, very little has been done on the clay alteration which overprints everything. In order to better understand this alteration, a study was performed on 45 samples using x-ray diffraction, Reitvelds analysis, formamide intercalation, scanning electron microscope and acid etching. XRD revealed that the two major clays at Cerro la Mina were halloysite (7Å and 10Å) and kaolinite which were identified in 30 and 34 of the 45 samples respectively. Other clays included alunite in 12 samples, dickite in eight samples, illite in 15 samples, pyrophyllite in six samples and an unknown white clay in one sample. Halloysite (7Å and 10Å) and kaolinite were found together in 28 of the 45 samples. Twelve samples were then analyzed using the SEM and showed the presence of tubular and spheroidal halloysite as well as platy kaolinite throughout each sample. Acid etching of 21 samples revealed that feldspars at Cerro la Mina are secondary in origin.

The association of both halloysite 7Å and 10Å with minerals including quartz, alunite,
dickite, kaolinite, gypsum, and jarosite suggests a supergene overprinting of the primary mineralogy. However, the association of orthoclase, phlogopite, pyrite and molybdenite may indicate that some halloysite was formed as a result of hypogene processes.  The parent mineral subsequently altered to halloysite and kaolinite was most likely feldspar and not mica, with early formed halloysite later being converted to kaolinite. The Cerro la Mina prospect is interpreted to be a high-sulfidation epithermal deposit situated within a lithocap overtop a deeper porphyry copper system.