GEOLOGICAL SOCIETY OF THE PHILIPPINES

Geology, mineralogy and fluid inclusion analysis of the Lalab orebody, (Philex Mining Corporation) Sibutad, Zamboanga del Norte, Philippines

 

(F.A. Jimenez, Jr., G.P. Yumul, Jr. and V.B. Maglambayan)

 

National Institute of Geological Sciences

College of Science, University of the Philippines

Diliman, Quezon City

 

 

ABSTRACT

 

            The Sibutad gold deposit is a gold-bearing vein-type mineralization in Sibutad, Zamboanga del Norte, which was mined by the Philex Mining Corporation. The Sibutad deposit includes two main gold orebodies, named the Lalab and Larayan, and, at least seven smaller gold-bearing quartz vein prospects that are aligned along the NW-SE direction. The Lalab and Larayan orebodies were diamond-drilled and the combined mineable resource, as of December 1998, was 3.4 million tons with an average grade of 1.8 g/ton Au. This estimate is based on the cut-off grade of 0.5 g/ton Au and represents the gold reserves under the 81.8%-owned Philex Gold Philippines, Inc. The Lalab orebody has the greater economic potential, since it contains 3.3 million tons, which is more than 95% of the total mineable reserves, with an average grade of 1.8 g/ton Au. The Lalab orebody is divided into two mineralized areas, namely the West Lalab, which is located at the northwestern portion of the orebody, and the Main Lalab, which occupies the southern portion. The focus of this paper is the study on the field geologic relations between the host rocks and hydrothermal-related geologic features, like the hydrothermal breccia bodies, siliceous deposit, wallrock alteration, mineralization and the temperature and composition of the hydrothermal solution.

 

The rocks identified in the Sibutad area and vicinity are composed of the volcanic rocks sequence, the Malindang Volcanics (Fig. 1). The volcanic rocks identified in the study area are divided into the lower and the upper members, which were intruded by an andesite porphyry body. The lower member is made up of the andesite flow and bedded dacite tuff/tuffite, whereas the unconformably overlying upper member is composed of basaltic to andesitic volcanic breccia and tuff breccia. In Lalab, hydrothermal alteration and mineralization occur in the andesite flow, which is of two types, the aphanitic and the porphyritic andesites. Aside from these two lithologies, pit mapping and drillcore logging have delineated hydrothermal breccia bodies and siliceous deposit. Several features of the hydrothermal breccia, such as the milky quartz and amorphous silica clasts, were used to infer the occurrence of four distinct hydrothermal events. Arranged from early to late, these are the pre-breccia wallrock alteration, formation of hydrothermal breccia bodies, post-breccia wallrock alteration and formation of cross-cutting milky quartz veins and veinlets. The hydrothermal solutions that produced the pre-breccia and post-breccia wallrock alterations and the milky quartz veins and veinlets were equated to Stage I, Stage II and Stage III fluids, respectively. These hydrothermal solutions were studied in the context of ore mineralization and fluid inclusion microthermometry.

 

Mineralization related to the Stage I fluid is characterized by the occurrence of pyrite, sphalerite+galena, chalcopyrite and bornite. Stage II mineralization is composed of pyrite, chalcopyrite, bornite and minor amounts of gold. Sphalerite is rare to absent in Stage II. Stage III saw the deposition of pyrite, bornite, chalcopyrite and gold. Bornite predominates over chalcopyrite, and sphalerite is absent, in Stage III. Gold mineralization in stages II and III is confirmed by the gold assays of >0.5 g/ton Au in post-breccia altered rocks, and >1.0 g/ton Au if these rocks contain milky quartz veins and veinlets. The predominance of sphalerite and galena over chalcopyrite in Stage I suggests that chloride is the principal metal-ligand in the Stage I fluid. On the other hand, the predominance of bornite over chalcopyrite, the absence of sphalerite and the deposition of gold in Stage III points to bi-sulphide metal-ligand in the Stage III fluid. There is thus a shift from chloride metal-ligand in Stage I to bi-sulphide metal-ligand in Stage III.

 

Fluid inclusion analyses indicate that the Stage I fluid has temperature regime greater than 238C, the Stage II fluid within 210C to 235C and the Stage III fluid within 186C to 215C. The salinity of the fluids for the three stages range from 1.0 and 2.0 eq. wt. % NaCl. This decrease in temperature of the fluid from Stage I through Stage II to Stage III at nearly uniform salinity range suggests simple cooling process. The locations of fluid inclusions that were trapped in boiling condition suggest depths from 225m to 400m below the paleosurface. With respect to the estimated paleosurface, hydrothermal activities and mineralization in Lalab occurs, at least, 800m below.

 

Geology, mineralogy and fluid inclusion data for the Lalab orebody point to its epithermal character, as shown by its shallow occurrence, from the paleosurface to depth of around 800m, and temperature regime between 186C and, at least 250C. The style and mineralogy of wallrock alteration, which is suggestive of precipitation from reduced, near-neutral pH solution, the boiling-indicator bladed texture and the low-salinity composition of the hydrothermal fluid further favors the adularia-sericite classification for the Lalab orebody. The ore-bearing fluid was apparently carrying and transporting gold in the form of a bi-sulphide complex. The mechanisms that have contributed to the precipitation of gold from the solution are boiling, followed by simple cooling.

 
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