Elaboración de un marco conceptual

Three hand-specimens with abundant monazite and xenotime grains were selected from two drill cores (RCGM 88 and 93) that transect the ore breccia at depth ca. 98 and 127 meters, respectively. Two generations of hydrothermal monazite and xenotime were identified in the ore breccia, each type being distinct by crystal morphology, chemistry and U-Pb isotopic composition. These are referred to as type (I) and type (II) monazite and xenotime, respectively. A third group of monazite (III) was defined on ground of distinct textural characteristics and comprises “composite” crystals that are commonly composed of a monazite (Mnz-I) core with overgrowth of Mnz-II monazite. The typical morphology and mode of occurrence of the individual types is shown in Figure 9.1.

Monazite and xenotime of type I (Mnz-I, Xn-I) form generally large (200 to 350 m) idiomorphic porphyroblasts that are primarily enclosed in chalcopyrite and pyrrhotite matrix. Xn-I is typically associated with Mnz-I either attached to Mnz-I grain boundaries forming characteristic coarse-grained monazite-xenotime pairs, or as small inclusions (~50 microns) within Mnz-I grains (Figure 9.1a-c). Mnz-I is on average 3-4 orders of magnitude more abundant than xenotime Xn-I in the studied ore zones. Mnz-I is present in form of equant to prismatic crystals with either sub-rounded or embayed grain boundaries. Xn-I tends to form dipyramidal crystals, particularly when attached to Mnz-I (Figure 9.1a-c). Coexisting Mnz-I and Xn-I grains appear to be rather optically homogeneous under plane polarized light; however they frequently show distinct zonation in electron microprobe (EMP) backscattered electron imaging (BSE) as well as X-ray mapping. The zoning patterns include three broad types such as concentric, patchy, and intergrowth-like zonation. They are virtually undistorted with well developed internal crystal faces parallel to grain boundaries and are interpreted to present primary growth zones (Figure 9.1c).

99

Figure 9.1 High-contrast backscattered electron (BSE) images showing (a-b) idiomorphic type I monazite (Mnz-I) and xenotime I (Xn-I) grains intergrown in chalcopyrite-pyrrhotite matrix, (c) pair of coarse-grained, euhedral monazite and xenotime porphyroblasts of type I exhibiting typical growth zonation, (d) xenomorphic type II monazite (Mnz-II) aggregates associated with Fe–Mg clinoamphibole, (e) weak concentric zonation of Mn-II clusters, and (f) typical occurrence of composite monazite (Mnz-III) grains intergrown with Fe-Mg clinoamphiboles.

100 In monazite Mnz-I, the growth-zoning pattern is comparable to that in the octaedron crystal structure. Similarly, in xenotime Xn-I the fine-scale, oscillatory zoning patterns occur as alternating light gray and dark zones parallel to rational crystal faces. The delicate zoning patterns for intergrown Mnz-I and Xn-I grains implicate textural equilibrium between the two phases and suggests coprecipitation with the sulfide matrix.

Monazite and xenotime of type II (Mnz-II, Xn-II) are typically associated with hydrous gangue minerals and occur typically as clusters interstitial to Fe-Mg clinoamphibole (Figure 9.1d,e). In contrast to xenotime grains of type I, Xn-II appears to be very rare, and only one small (ca. 30 m) grain could be identified. BSE imaging of Mnz-II shows that these grains lack the prismatic form, typical of Mnz-I, and are homogeneous or show only a weak internal concentric zoning (Figure 9.1d,e). BSE imaging on one single Xn II grain revealed a complex texture with multiple dark and light zones. Composite monazite (Mnz-III) grains occur always in association with single Mnz-II grains and Fe-Mg clinoamphibole. In some instances, composite monazite grains preserve partially monoclinic crystal geometry which is characteristic of Mnz-I (Figure 9.1f). The texture of the Mnz-II and the composite monazite grains suggest that, primary Mnz-I is replaced by secondary Mnz-II neoblasts and in places forms “composite” grains with inherited Mnz-I cores and Mnz-II overgrowths.

9.2 Mineral chemistry

A total of 18 LA-ICPMS analyses were performed on Mnz-I grains, 6 on Mnz-II, and 4 on Xn-I grains. Both monazite types have the nominal composition (LREEPO₄) and the mole fraction of the light rare earth elements (primarily Ce+La+Nd) ranges from 0.81 to 0.93 % of the total cation proportions (excluding P), with Ce averaging at 0.49 %. In addition, Y concentration is similar in both types varying between 0.01 and 0.03 cations/4 oxygens (Tables 12.49 and 12.50).

In Mnz-I the chondrite-normalized (C1) REE patterns are characterized by moderate lowering from the LREE to MREE (LaCN/GdCN = 2.0-5.5), a small positive Eu anomaly (Eu/Eu*= 1.03-1.49) and steep fall from the MREE to the HREE (Gd_CN/Lu_CN = 550-1500; Figure 9.2a). U/Th ratios vary between 1 and 20, while Th concentrations are between 0.01 and 0.1 wt.% and U contents in the range 0.06-0.22 wt.% (Table 12.49). The very low Th and U contents (<1 wt.%) and the absence of an Eu anomaly suggest a hydrothermal origin for the phosphates (Schandl and Gorton 2004). Concentration profiles from the rims to the centers of grains showed that the compositional heterogeneity observed from BSE imaging is related to minor variations in the LREE amounts. Bright zones at the outer parts of grains frequently observed with BSE imaging are related to enrichment of Ce and La, and to lesser extend Gd, and concomitant depletion of Nd and Sm.

In xenotime of type I (Xn-I) the mole fraction of Y is relatively uniform with an average value of 0.70 cations/4 oxygens (Table 12.51). Xenotime (Xn-I) is characterized by steep increase of chondrite-normalized abundances from the LREE to MREE (GdCN/LaCN = 1440-

101 4050), and rather flat patterns from the MREE to the HREE (GdCN/LuCN= 1.2-2.3). There is no distinct Eu anomaly, with Eu/Eu* varying between 0.96 and 1.25 (Figure 9.2b). U/Th ratios vary between 18 and 440, Th concentrations are between 1.5 and 55 ppm, and U is in the range 300-1966 ppm. Zr contents are between 10 and 30 ppm (Table 12.51). Concentration profiles from rim to center of xenotime grains show that the compositional heterogeneity observed from BSE imaging is related to minor variations in Y concentrations, as well as Gd, and Dy.

Figure 9.2 Chondrite (C1) normalized REE patterns of single grains (values after McDonough

and Sun 1989) (a) of monazite I (Mnz-I) and monazite II (Mnz-II), and (b) xenotime I (Xn-I).

Figure 9.3 Plot of U vs Th (wt.%) concentrations of single monazite grains from both monazite types (I and II). Monazite II can be distinguished from type I by lower U and higher Th amounts.

Monazite II also defines a rather flat trend of chondrite (C1) normalized abundances from La to Gd (3.8-7.3), but is distinct by less pronounced decreases from Gd to Lu (412-840) and small negative Eu anomalies (Eu/Eu*= 0.6-0.9). Th concentrations are distinctly higher (0.15-0.29 %) and U contents are lower (0.03-0.06), with U/Th ratios ranging from 0.14 to

102 0.29. U/Th ratios are distinctly lower than those found in monazite I (Figure 9.3; Table 12.50). Other chemical differences are in the Si concentrations which is constantly higher in monazite type II, while type I has higher Ca.